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Tunnel

Tunnel

]] A tunnel is an underground passage. When designed for use by traffic, it may be called an underpass. The definition of what constitutes a tunnel is not universally agreed upon. However, in general tunnels have a ratio of the length of the passage to the width of at least 2 to 1. In addition, they should be completely enclosed on all sides (save the openings) for the length of the covered area. A tunnel may be for pedestrians and/or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some are constructed purely for carrying water (for consumption, for hydroelectric purposes or as sewers), while others carry other services such as telecommunications cables. There are even tunnels designed as wildlife crossings for European badgers and other endangered species. In the UK a pedestrian tunnel or other underpass beneath a road is called a "subway". This term was also used in the past in the US, but is now used to refer to underground mass transit systems, generally called metros outside the US. The central part of a metro or subway network is usually built in tunnels. To allow non-level crossings, some lines are in deeper tunnels than others. At metro stations there are usually also pedestrian tunnels from one platform to another. Often, ground-level railway stations also have one or more pedestrian tunnels under the railway to enable passengers to reach the platforms without having to walk across the tracks.

Types of tunnels

Tunnels are dug in various types of materials, from soft clays to hard rocks, and the method of excavation depends on the type of soil. When digging soft clays the New Austrian Tunneling method might be applied. When digging in weak rocks a tunnel boring machine (TBM) is often used. In hard rocks, blasting may be deemed most suitable, as in the Norwegian tunneling method. Various combinations of these methods and many others are also possible. There are also several approaches to underwater tunnels, for instance an immersed tube as in Sydney Harbour.

Construction

Sydney Harbour]] Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over. Strong supporting beams are necessary to avoid the danger of the tunnel collapsing. Shallow tunnels are often of the cut-and-cover type (if under water of the immersed-tube type), while deep tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are possible. Tunnel-boring machines (TBMs) can be used to automate the entire tunneling process. There are a variety of TBMs that can operate in a variety of conditions. One type of TBM, called an earth-pressure balance machine, can be used deep below the water table. This pressurizes the cutter head with either fluid or air in order to balance the water pressure. As a result operators of the TBM must go through decompression chambers, much like divers.
The biggest TBM built was operated to drill the tunnel as part of the High Speed Raillink South in the Netherlands. Its diameter is 14.85 m. [http://www.hslzuid.nl/hsl/bouw/ment/Boortunnel_Groene_Hart/index.jsp Tunnel Groene Hart (the Netherlands)] Very effective is the New Austrian Tunneling Method (NATM), which was developed in the 1960s. The main idea of this important method is to use the geological stress of the surrounding rock mass to stabilize the tunnel itself.
Based on geotechnical measurements, an optimal cross section is computed. The excavation is immediately protected by thin shotcrete, just behind the TBM. This creates a natural load-bearing ring, which minimizes the rock's deformation. By special monitoring the NATM method is very flexible, even at surprising changes of the geomechanical rock consistency during the tunneling work. The measured rock properties lead to appropriate tools for tunnel strengthening. In the last decades also soft ground excavations up to 10 km became usual. Other tunneling methods are:
- Drill and blast
- Lunardi Method
- Slurry-shield machine
- Wall-cover construction method.

Choice of tunnels vs. bridges

For water crossings, a tunnel is generally more costly to construct than a bridge. However, navigational considerations may limit the use of high bridges or drawbridge spans when intersecting with shipping channels at some locations, necessitating use of a tunnel. Additionally, bridges usually require a larger footprint on each shore than tunnels; in areas with particularly expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in tunnels' favor. Examples of water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City, and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia and the [http://www.westerscheldetunnel.nl Westerscheldetunnel]. Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link.

Very short tunnels

A short tunnel can be a convenient way to build an overbridge. Very short tunnels resemble overbridge. For example, the Croom Tunnel on the Nowra, New South Wales railway line.

Artificial Tunnels

Overbridges can sometimes be built by covering a road or river or railway with brick or still arches, and then levelling the surface with earth. In railway parlence, a surface-level track which has been built or covered over is normally called a covered way. Snowsheds are a kind of artificial tunnel built to protect a railway from avalanches of snow. Similarly the Coal Cliff, New South Wales steel tunnel, on the Nowra, New South Wales railway line.

Examples of tunnels

In history

Nowra, New South Wales, mid 19th century]]
- Eupalinos Tunnel on the island Samos (Greece). Built 520 BC by the greek engineer Eupalinos. Eupalinos organised the work so that the tunnel was begun from both sides of the hill and the two teams met in the middle. The estimates for the time required range from 5 to 15 years: the mountain is solid limestone and one has to suppose that many of the slaves doing the work died. The tunnel's existence was recorded by Herodotus (as was the mole and harbour, and the third wonder of the island, the great temple to Hera, thought by many to be the largest in the Greek world). The precise location of the tunnel was only re-established in the 19th century by German archaeologists. The tunnel proper is 1030 metres - 3432 feet - long and visitors can still enter it [http://homepages.cwi.nl/~aeb/math/samos/ Eupalinos tunnel].
- Sapperton Tunnel on the Thames & Severn Canal in England, dug through hills, which opened in 1789, was 3.5 km long and allowed ship transport of coal. Above it runs the Sapperton Long Tunnel which carries the "Golden Valley" railway line between Swindon and Gloucester.
- Box Tunnel in England, which opened in 1841, was the longest railway tunnel in the world at the time of construction. It was dug and has a length of 2.9 km.
- The Thames Tunnel, built by Marc Isambard Brunel and his son Isambard Kingdom Brunel and opened in 1843, was the first underwater tunnel and the first to use a tunnelling shield.
- The Cobble Hill Tunnel and Murray Hill Tunnel in New York City are the world's oldest railway tunnels lying below streets, roofed over in 1850 and the 1850s, respectively.
- The oldest sections of the London Underground were built using the cut-and-cover method in the 1860s. The Metropolitan, Hammersmith & City, Circle and District lines were the first to prove the success of a metro or subway system. See also the History of Rapid transit.

The longest

Rapid transit]]
- A new St. Gotthard Rail Tunnel with a length of 57 km (36 miles) is currently under construction.
- The Seikan Tunnel in Japan is the longest rail tunnel in the world at 53.9 km (33.4 miles), of which 23.3 km (14.5 miles) is under the sea.
- The Channel Tunnel between France and England under the English Channel is the second-longest, with a total length of 50 km (31 miles), of which 39 km (24 miles) is under the sea.
- The Lærdal Tunnel in Norway from Lærdal to Aurland is the world's longest road tunnel, intended for cars and similar vehicles, at 24.5 km.
- The St. Gotthard Tunnel from Göschenen to Airolo in Switzerland, opened on September 5, 1980, is the world's longest highway tunnel at 16.32 km (10.14 miles).
- The North Cape Tunnel in northern Norway, connecting the island of Magerøya with the mainland, was the world's longest undersea road tunnel when opened in 1999, at about 7 km. It reaches a depth of 212 m below sea level.

Notable

- The Lincoln Tunnel between New Jersey and New York is one of the busiest vehicular tunnels in the world.
- Williamson's tunnels in Liverpool are probably the largest underground folly in the world.
- New York City Water Tunnel No. 3[http://www.nyc.gov/html/dep/html/news/3rdtunnel.html], started in 1970, has an expected completion date of 2020.

Other uses

Excavation techniques, as well as the construction of underground bunkers and other habitable areas, are often associated with military use during armed conflict, or civilian responses to threat of attack, or a means of protection from attackers.

External links

- Directory of [http://home.no.net/lotsberg/index.html the world's longest tunnels] by category
- Category:Subterranea Category:Bridges ja:トンネル simple:Tunnel

Pedestrian

, Australia]] A pedestrian is a person travelling on foot, whether walking or running. In modern times, the term mostly refers to someone walking on a road but this was not the case historically.

History

During the eighteenth and nineteenth centuries, pedestrianism was a popular spectator sport just as equestrianism still is. One of the most famous pedestrians of the day was Captain Robert Barclay Allardice, known as "The Celebrated Pedestrian", of Stonehaven. His most impressive feat was to walk 1 mile every hour for 1000 hours, which he achieved between the 1st of June and the 12th of July, 1809. This feat captured the imagination of the public, and around 10,000 people came to watch over the course of the event. During the rest of the nineteenth century attempts to repeat this particular athletic challenge were made by many pedestrians including the renowned Ada Anderson who developed it further and walked a quarter-mile in each quarter-hour over the 1,000 hours. Since the nineteenth century, interest in pedestrianism has dropped. Although it is still an Olympic sport, it fails to catch public attention in the way that it used to. However, pedestrians are still carrying out major walking feats such as the popular Land's End to John o' Groats walk, in the United Kingdom, or traversal of North America from coast to coast. These feats are often tied to charitable fundraising and have been achieved by celebrities such as Sir Jimmy Savile or Ian Botham as well as by people not otherwise in the public eye.

Health and Environment

Regular walking is very important for both a person's health and the natural environment. Obesity and related medical problems can be effectively prevented and/or cured by moving on foot on a daily basis. The widespread habit of taking the car for short grocery trips significantly contributes to both obesity and the emissions problem, as internal combustion engines are extremely inefficient and highly polluting during their first minutes of operation (engine cold start). General availability of public transportation encourages walking, as it won't take one directly to one's doorstep.

Roads

Nowadays, roads often have a designated footpath attached especially for pedestrian traffic, called the sidewalk in American English and the pavement in British English. There are also footpaths not associated with a road which are used purely by pedestrians, particularly ramblers, hikers or hill-walkers and there are roads not associated with a footpath. Such footpaths in mountainous or forested areas are called trails. On some of the latter, pedestrians share the road with horses and vehicles whilst on others they are forbidden from using the road altogether. Also some shopping streets are for pedestrians only. Some roads have special pedestrian crossings. A bridge solely for pedestrians is a footbridge.

Pedestrianisation

Efforts are underway by pedestrian advocacy groups to restore pedestrian access to new developments, especially to counteract newer developments where 20 to 30 percent do not include sidewalks. Some activists advocate large auto-free zones where pedestrians only or pedestrians and some non motorised vehicles are allowed. Many urbanists have extolled the virtues of pedestrian streets in urban areas. Many urban streets in the USA lack street lighting (lamp poles), based on the reasoning that cars have headlights to illuminate their own way. This policy severely restricts or effectively prohibits pedestrian traffic and contributes to excessive car use on short distance trips. In contrast pedestrian traffic is officially encouraged in some parts of the European Union and construction or separation of dedicated walking routes receives a high priority in most large European city centres, often in conjunction with public transport enhancements. In Copenhagen the world's longest pedestrian shopping area, the Strøget, has been developed over the last 40 years principically due to the work of Danish architect Jan Gehl. The word pedestrian also has a figurative meaning of "unimaginative" or "ordinary." Ex: She wrote pages and pages of pedestrian prose.

External links

- [http://www.ultramarathonworld.com/uw_archive/m18ja00a.html Ultramarathons]
- [http://www.lehigh.edu/dmd1/public/www-data/kelly.html Early Pedestrians in North America]
- [http://www.preservenet.com/politics/PedsRights.html Pedestrian Advocacy Groups]
- [http://www.transalt.org/campaigns/reclaiming/ Transportation Alternatives: Pedestrian Advocacy]
- [http://americawalks.org/ America Walks] Category:Transportation ja:歩行者

Motor vehicle

A motor vehicle can be:
- an automobile
- a motorcycle
- a motorized bicycle Many rules of the road / traffic code are usually the same for both:
- where one can and cannot drive
- speed limit
- right of way
- license plate A helmet is specific for a motorcycle. The driving license may be different. Category:Vehicles

Hydroelectric

Hydroelectricity is a form of hydropower used to produce electricity. Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator. Less common variations make use of water's kinetic energy or undammed sources such as tidal power. Hydroelectricity is a renewable energy source. The energy extracted from water depends not only on the volume but on the difference in height between the source and the water's outflow. This height difference is called the head. The amount of potential energy in water is directly proportional to the head. For this reason, it is advantageous to build dams as high as possible to produce the maximum electrical energy. While many hydroelectric schemes supply public electricity networks, some projects were created for private commercial purposes. For example, aluminium processing requires substantial amounts of electricity, and in Britain's Scottish Highlands there are examples at Kinlochleven and Lochaber, designed and constructed during the early years of the 20th century. Similarly, the 'van Blommestein' lake, dam and power station were constructed in Suriname to provide electricity for the Alcoa aluminium industry. In most parts of Canada (the provinces of British Columbia, Manitoba, Ontario, Quebec and Newfoundland and Labrador) hydroelectricity is used so extensively that the word "hydro" is used to refer to any electricity delivered by a power utility. The government-run power utilities in these provinces are called BC Hydro, Manitoba Hydro, Hydro One (formerly "Ontario Hydro"), Hydro-Québec and Newfoundland and Labrador Hydro respectively. Hydro-Québec is the world's largest hydroelectric generating company, with a total installed capacity (2005) of 31,512 MW

Importance

Hydroelectric power, using the potential energy of rivers, now supplies 20% of world electricity. Norway produces virtually all of its electricity from hydro, while Iceland produces 83% of its requirements (2004), Austria produces 67 % of all electricity generated in the country from hydro (over 70 % of its requirements). Canada is the world's largest producer of hydro power and produces over 70% of its electricity from hydroelectric sources. Apart from a few countries with an abundance of it, hydro capacity is normally applied to peak-load demand, because it can be readily stored during off-peak hours (in fact, pumped-storage hydroelectric reservoirs are sometimes used to store electricity produced by thermal plants for use during peak hours). It is not a major option for the future in the developed countries because most major sites in these countries having potential for harnessing gravity in this way are either being exploited already or are unavailable for other reasons such as environmental considerations.

Advantages and disadvantages

pumped-storage hydroelectric reservoirs The chief advantage of hydro systems is elimination of the cost of fuel. Hydroelectric plants are immune to price increases for fossil fuels such as oil, natural gas or coal, and do not require imported fuel. Hydroelectric plants tend to have longer lives than fuel-fired generation, with some plants now in service having been built 50 to 100 years ago. Labor cost also tends to be low since plants are generally heavily automated and have few personnel on site during normal operation. Hydroelectric plants generally have small to negligible emissions of carbon dioxide and methane due to reservoir emissions, and emit no sulfur dioxide, nitrogen oxides, dust, or other pollutants associated with combustion. Since the generating units can be started and stopped quickly, they can follow system loads efficiently, and may be able to reshape water flows to more closely match daily and seasonal system energy demands. Hydroelectric plants with reliable hydrological histories are dispatchable and can be considered firm capacity. Consequently, in normal water years hydroelectric plants designed for a firm load will have a useful amount of surplus energy that may be exportable if transmission is available. Pumped storage plants currently provide the most significant means of storage of energy on a scale useful for a utility, allowing low-value generation in off-peak times (which occurs because fossil-fuel plants cannot be entirely shut down on a daily basis) to be used to store water that can be released during high load daily peaks. Operation of pumped-storage plants improves the daily load factor of the generation system. Reservoirs created by hydroelectric schemes often provide excellent leisure facilities for water sports, and become tourist attractions in themselves. Multi-use dams installed for irrigation, flood control, or recreation, may have a hydroelectric plant added with relatively low construction cost, providing a useful revenue stream to offset the cost of dam operation. In practice, the utilization of stored water is sometimes complicated by demand for irrigation which may occur out of phase with peak electricity demand. Times of drought can cause severe problems, since water replenishment rates may not keep up with desired usage rates. Minimum discharge requirements represent an efficiency loss for the station if it is uneconomic to install a small turbine unit for that flow. Concerns have been raised by environmentalists that large hydroelectric projects might be disruptive to surrounding aquatic ecosystems. For instance, studies have shown that dams along the Atlantic and Pacific coasts of North America have reduced salmon populations by preventing access to spawning grounds upstream, even though most dams in salmon habitat have fish ladders installed. Salmon smolt are also harmed on their migration to sea when they must pass through turbines. This has led to some areas barging smolt downstream during parts of the year. Turbine and power-plant designs that are easier on aquatic life are an active area of research. Generation of hydroelectric power can also have an impact on the downstream river environment. First, water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks. Second, since turbines are often opened intermittently, rapid or even daily fluctuations in river flow are observed. In the Grand Canyon, the daily cyclic flow variation caused by Glen Canyon Dam was found to be contributing to erosion of sand bars. Dissolved oxygen content of the water may change from preceding conditions. Finally, water exiting from turbines is typically much colder than the pre-dam water, which can change aquatic faunal populations, including endangered species. The reservoirs of hydroelectric power plants in tropical regions may produce substantial amounts of methane and carbon dioxide. This is due to plant material in newly flooded and re-flooded areas being inundated with water, decaying in an anaerobic environment, and forming methane, a very potent greenhouse gas. The methane is released into the atmosphere once the water is discharged from the dam and turns the turbines. According to the World Commission on Dams report, where the reservoir is large compared to the generating capacity (less than 100 watts per square metre of surface area) and no clearing of the forests in the area was undertaken prior to impoundment of the reservoir, greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant [http://www.newscientist.com/article.ns?id=dn7046]. In boreal reservoirs of Canada and Northern Europe, however, greenhouse gas emissions are typically only 2 to 8 percent of any kind of conventional thermal generation. Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population. Additionally, historically and culturally important sites can be lost, such as the Three Gorges Dam project in China, the Clyde Dam in New Zealand and the Ilisu Dam in Southeastern Turkey. Some hydroelectric projects also utilize canals, typically to divert a river at a shallower gradient to increase the head of the scheme. In some cases, the entire river may be diverted leaving a dry riverbed. Examples include the Tekapo and Pukaki Rivers.

Hydro-electric facts

Oldest

- Cragside, Rothbury, England completed 1870, Water commercial service at Minneapolis.
- Duck Reach, Launceston, Tasmania. Completed 1895. The first publicly-owned hydro-electric plant in the Southern Hemisphere. Supplied power to the city of Launceston for street lighting.
- Decew Falls 1, St. Catharines, Ontario, Canada completed 25 August 1898. Owned by Ontario Power Generation. Four units are still operational. Recognised as an IEEE Milestone in Electrical Engineering & Computing by the IEEE Executive Committee in 2002.

Largest hydro-electric power stations

IEEE The La Grande Complex in Quebec, Canada, is the world's largest hydroelectric generating system. The eight generating stations of the complex have a total generating capacity of 16,021 MW. The Robert-Bourassa station alone has a capacity of 5,616 MW. A nineth station (Eastmain-1) is currently under construction and will add 480 MW to the total. An additional project on the Rupert River, currently undergoing environmental assessments, would add two stations with a combined capacity of 888 MW.

Fully operational

Name	Country	Completed	Max Generation	Annual Production
Itaipú	Brazil/Paraguay	1983	12,600 MW	93.4 TW-hours
Guri	Venezuela	1986	10,200 MW	46 TW-hours
Grand Coulee	United States	1942/1980	6,809 MW	22.6 TW-hours
Sayano Shushenskaya	Russia	1983	6,400 MW
Robert-Bourassa	Canada	1981	5,616 MW
Churchill Falls	Canada	1971	5,429 MW	35 TW-hours

These are ranked by maximum power.

In progress

- Three Gorges Dam, China, First power July 2003,Scheduled completion, 2009, 18,200 MW

Countries with the most hydro-electric capacity

- Canada, 341,312 GWh (66,954 MW installed)
- USA, 319,484 GWh (79,511 MW installed)
- Brazil, 285,603 GWh (57,517 MW installed)
- China, 204,300 GWh (65,000 MW installed)
- Russia, 160,500 GWh (44,000 MW installed)
- Norway, 121,824 GWh (27,528 MW installed)
- Japan, 84,500 GWh (27,229 MW installed)
- India, 82,237 GWh (22,083 MW installed)
- France, 77,500 GWh (25,335 MW installed) These are 1999 figures and include pumped-storage hydroelectricity schemes.

References

#[http://www.newscientist.com/article.ns?id=dn7046 New Scientist report on greenhouse gas production by hydroelectric dams.] #[http://www.waterpowermagazine.com/story.asp?sectioncode=165&storyCode=2019676 International Water Power and Dam Construction Venezuela country profile] #[http://www.waterpowermagazine.com/story.asp?sectionCode=165&storyCode=2019652 International Water Power and Dam Construction Canada country profile] #[http://www.springer.com/sgw/cda/frontpage/0,,1-10006-22-35070329-0,00.html Tremblay, Varfalvy, Roehm and Garneau. 2005. Greenhouse Gas Emissions - Fluxes and Processes, Springer, 732 p.]

External links

- [http://www.cus.net/renewableenergy/subcats/hydroelectric/hydroelectric.html Hydroelectric power]
- [http://www.dams.org/ World Commission on Dams report on environmental and social effects of large dams, including discussion of greenhouse gas emissions]
- [http://www.freeenergynews.com/Directory/Hydro/ Hydroelectricity] - Water potential powered systems, focusing on non-impactive small hydro. (FreeEnergyNews.com)
- [http://www.freeenergynews.com/Directory/River/ River Energy] - river turbine systems, not dam. (FreeEnergyNews.com) Category:Renewable energy Category:Energy conversion Category:Energy storage Category:Alternative energy fuck ja:水力発電 simple:Hydroelectricity

Sewers

Sewers transport wastewater from buildings to treatment facilities. Sewers are pipelines that connect buildings to horizontal 'mains'. The sewer mains often connect to larger mains, and then to the wastewater treatment site. Vertical pipes, called manholes, connect the mains to the surface. Sewers are generally gravity powered, though pumps may be used if necessary. Storm sewers (also storm drains) are large pipes that transport storm water runoff from streets to natural bodies of water, to avoid street flooding. When the two systems are operated separately, the sewer system that is not the set of storm drains is called a sanitary sewer. Catchbasins are immediately below the vertical pipes connecting the surface to the storm sewers. While sewer grates covering the vertical pipes prevent large objects from falling into the sewer system, the grates are spaced far enough apart that many small objects can fall through. The area immediately below the catchbasin "catches" such detritus. Water from the top of the catchbasin drains into the sewer proper. The catchbasin serves much the same function as the "trap" in household wastewater plumbing in trapping objects. Unlike the trap, the catchbasin does not necessarily prevent sewer gases such as hydrogen sulfide and methane from escaping. Catchbasins contain stagnant water and can be used by mosquitoes for breeding. Catchbasins require regular cleaning to remove the trapped debris. Municipalities typically have large vacuum trucks that clean out catchbasins. Storm sewer water may be treated or not, depending on jurisdiction. Treatment helps purify the storm water before being restored to a natural body of water. Storm water may become contaminated while running down the road or other impervious surface, or from lawn chemical runoff, before entering the sewer. It is a good idea to separate storm sewers from waste sewers because the huge influx of water during a rainstorm can overwhelm the treatment plant, resulting in untreated sewage being discharged into the environment. Washington DC and other cities with older combined systems have this problem after every heavy rain. However, completely separating storm sewers from sanitary sewers often means no treatment of stormwater which is not desirable either, as the first flush from storm runoff can be extremely dirty. Runoff into storm sewers can be minimized by including Sustainable Urban Drainage Systems in to municipal plans. Eaves troughs should not discharge directly into the storm sewer system but rather onto the ground where it has a chance to soak into the soil. Where possible, storm water runoff should be directed to unlined ditches before flowing into the storm sewers, again to allow the runoff to soak into the ground. Separation of undesired runoff can be done within the storm sewer system, but such devices are new to the market and can only be installed with new development or during major upgrades. They are referred to as oil-grit separators (OGS) or oil-sediment separators (OSS). They consist of a specialized manhole chamber, and use the water flow and/or gravity to separate oil and grit. See also sewage treatment, infiltration gallery

History

The earliest covered sewers uncovered by archeologists are in the regularly planned cities of the Indus Valley Civilization. In ancient Rome, the Cloaca Maxima, considered a marvel of engineering, disgorged into the Tiber. In medieval European cities, small natural waterways used for carrying off wastewater were eventually covered over and functioned as sewers. London's Fleet is such a system. Open drains along the center of some streets were known as 'kennels' ('canals'). The 19th-century brick-vaulted sewer system of Paris offers tours for tourists.

Sewers in fiction

The theme of traveling through, hiding, or even residing in sewers is a common cliché in fantasy role-playing games, video games and, to a lesser extent, movies. Some titles that include this cliché are the Mario series, Deus Ex, Dark Cloud 2, The Matrix, Resident Evil 2, The Shawshank Redemption, Teenage Mutant Ninja Turtles and many materials related to the influential Dungeons and Dragons game. The darkness, maze-like layout, separation/connection with the city overhead (not to mention the slime) create a perfect environment for fantasy adventure. The odors and health concerns that would surround sewer travel in real life are often not addressed or are downplayed in these titles. A well-known urban legend, known as Sewer alligator, is that of giant alligators or crocodiles residing in sewers, especially of major metropolitan areas. The sewers under Vienna serve as the setting for the climactic scene in the film The Third Man. (These sequences are echoed in the American film noir, He Walks by Night, starring Richard Basehart. Sewers are also used in the novel Les Misérables as a setting for escaping and pursuit following a lost battle. The Thomas Pynchon novel, 'V', features extended passages in which one of the protagonists, Benny Profane, works with a fictional New York City task force to track alligators in the city sewers. His goal is to bag the great albino alligator, reputed to inhabit the system. This literary conceit grows from the persistent urban legend that baby pet alligators, flushed down toilets by tourists returning from Florida, continue to live and flourish in the pipes below. The image of the sewer recurs in European culture as they were often used as hiding places or routes of escape by the scorned or the hunted, including partisans and resistance fighters in WWII. The only survivors from the Warsaw Ghetto made their final escape through city sewers. Some have commented that the imaginary prison engravings by Piranesi were inspired by the Cloaca Maximus.

Accidents

A sewer main in Guadalajara, Mexico had to be diverted down through a siphon to allow space for a metro railway to be built. The siphon allowed water and waste to pass, but not fumes. Petrol which spilled or leaked into the sewer on one side of the siphon was not able to easily escape to the safe exit on the other side, leading eventually to a build-up of fumes and an explosion killing hundreds. These explosions occurred in 1983 and most seriously on April 22, 1992. A trap is a U-shaped bend in a water conduit, as found on toilets, and wash basin outlets. Most of the time, traps are used to block the fumes, but not the waste and water.

Lessons learned

The sewer siphon should have had a second siphon over the metro tunnel to allow fumes to get from one side to the other, as if the Metro tunnel were not there.

External links

[http://www.sewerhistory.org/ Site on the history of sewers] ja:下水道 Category:Sewerage Category:Subterranea Category:Water streams Category:Construction

Telecommunications

Telecommunication refers to communication over long distances. In practice, something of the message may be lost in the process. Telecommunication covers all forms of distance and/or conversion of the original communications, including radio, telegraphy, television, telephony, data communication and computer networking. The elements of a telecommunication system are a transmitter, a medium (line) and possibly a channel imposed upon the medium (see baseband and broadband as well as multiplexing), and a receiver. The transmitter is a device that transforms or encodes the message into a physical phenomenon; the signal. The transmission medium, by its physical nature, is likely to modify or degrade the signal on its path from the transmitter to the receiver. The receiver has a decoding mechanism capable of recovering the message within certain limits of signal degradation. Sometimes, the final "receiver" is the human eye and/or ear (or in some extreme cases other sensory organs) and the recovery of the message is done by the brain (see psychoacoustics.) Telecommunication can be point-to-point, point-to-multipoint or broadcasting, which is a particular form of point-to-multipoint that goes only from the transmitter to the receivers. One of the roles of the telecommunications engineer is to analyse the physical properties of the line or transmission medium, and the statistical properties of the message in order to design the most effective encoding and decoding mechanisms. When systems are designed to communicate through human sensory organs (mainly those for vision and hearing), physiological and psychological characteristics of human perception must be taken into account. This has important economic implications and engineers must research what defects can be tolerated in the signal and not significantly degrade the viewing or hearing experience.

Examples of human (tele)communications

In a simplistic example, consider a normal conversation between two people. The message is the sentence that the speaker decides to communicate to the listener. The transmitter is the language areas in the brain, the motor cortex, the vocal cords, the larynx, and the mouth that produce those sounds called speech. The signal is the sound waves (pressure fluctuations in air particles) that can be identified as speech. The channel is the air carrying those sound waves, and all the acoustic properties of the surrounding space: echoes, ambient noise, reverberation. Between the speaker and the listener, there might be other devices that do or do not introduce their own distortions of the original vocal signal (for example a telephone, a HAM radio, an IP phone, etc.) The receiver is the listener's ear and auditory system, the auditory nerve, and the language areas in the listener's brain that will "decode" the signal into meaningful information and filter out background noise. All channels have noise. Another important aspect of the channel is called the bandwidth. A low bandwidth channel, such as a telephone, cannot carry all of the audio information that is transmitted in normal conversation, causing distortion and irregularities in the speaker's voice, as compared to normal, in-person speech.

External links

- [http://web.archive.org/web/20040413074912/www.ericsson.com/support/telecom/index.shtml Ericsson's Understanding Telecommunications] at archive.org (Ericsson removed the book from their site in Sep 2005)
- [http://www.carrieraccessbilling.com/telecommunications-glossary-a.asp Intec Telecom Systems' Telecom Dictionary]
- [http://www.mobile-phone-directory.org/Glossary/ Mobile Phone Directory Telecommunications Glossary]
- [http://www.tiaonline.org Telecommunications Industry Association (TIA)]
- [http://www.aronsson.se/hist.html Aronsson's Telecom History Timeline]
- [http://www.alcatel.com/atr Alcatel Telecommunications Review] Telecom magazine published since 1922
- [http://www.teleclick.ca Telecommunications Industry News]
- [http://www.bt.com BT] British Telecommunications company
- Category:Digital Revolution ms:Telekomunikasi ja:電気通信 th:โทรคมนาคม

European Badger

:Eurasian Badger

Endangered species

:For other uses, see endangered species (disambiguation). endangered species (disambiguation) due to extreme overhunting.]] An endangered species is a species whose population is so small that it is in danger of becoming extinct. Many countries have laws offering special protection to these species (forbidding hunting, banning development, etc. of their habitats ) to prevent their extinction. Only a few of the many truly endangered species actually make it to the lists and obtain legal protection. Many more species become extinct, or potentially will become extinct, without gaining public notice. Many of these laws are controversial. Typical areas of controversy include: criteria for placing a species on the endangered species list, and criteria for removing a species from the list once its population has recovered; whether restrictions on land development constitute a "taking" of land by the government, and the related question of whether private landowners should be compensated for the loss of use of their land; and obtaining reasonable exceptions to protection laws. A listing as an endangered species can backfire, as it makes a species more valuable and more desirable for collectors and poachers [http://news.bbc.co.uk/2/hi/science/nature/4013719.stm]. The conservation status of a species is an indicator of the likelihood of that endangered species continuing to survive. Many factors are taken into account when assessing the conservation status of a species; not simply the number remaining, but the overall increase or decrease in the population over time, breeding success rates, known threats, and so on. The best-known worldwide conservation status listing is the IUCN Red List, but many more specialized lists exist. The following conservation status categories are used in articles in this encyclopedia. They are loosely based on the IUCN categories.
- Extinct: the last remaining member of the species had died, or is presumed to have died beyond reasonable doubt. Examples: Thylacine, Dodo.
- Extinct in the wild: captive individuals survive, but there is no free-living, natural population. Examples: Dromedary, Przewalski's Horse.
- Critical or critically endangered: faces an extremely high risk of extinction in the immediate future. Examples: Ivory-billed Woodpecker, Arakan Forest Turtle
- Endangered: faces a very high risk of extinction in the near future. Examples: Kings holly, Pink fairy armadillo
- Vulnerable: faces a high risk of extinction in the medium-term. Examples: Cheetah, Bactrian Camel
- Secure or lower risk: no immediate threat to the survival of the species. Examples: Nootka Cypress, Llama The following lists are examples of endangered species. Note that because of varying standards for regarding a species as endangered, and the very large number of endangered species, these lists should not be regarded as comprehensive.

Endangered mammals

- Asian Elephant (Elephas maximus)
- Aye-aye (Daubentonia madagascariensis)
- Banteng (Bos javanicus)
- Bighorn Sheep (Ovis canadensis)
- Black-footed Ferret (Mustela nigripes)
- Blue Whale (Balaenoptera musculus)
- Bonobo (Pan paniscus)
- Burrowing Bettong (Bettongia lesueur nova)
- Common Chimpanzee (Pan troglodytes)
- Chinese River Dolphin (Lipotes vexillifer) and other river dolphins
- Eastern Gorilla (Gorilla beringei)
- Fin Whale (Balaenoptera physalus)
- Forest Elephant (Loxodonta cyclotis)
- Gelada (Theropithecus gelada)
- Giant golden-crowned flying fox (Acerodon jubatus)
- Giant Panda (Ailuropoda melanoleuca)
- Giant Pangolin (Manis gigantea)
- Golden Lion Tamarin (Leontopithecus rosalia)
- Gray bat (Myotis grisescens)
- Hawaiian Monk Seal (Monachus schauinslandi)
- Humpback Whale (Megaptera novaeangliae)
- Indian Pangolin (Manis crassicaudata)
- Indri (Indri indri)
- Island Fox (Urocyon littoralis)
- Key Deer (Odocoileus virginianus clavium)
- Kouprey (Bos sauveli)
- Leopard (Panthera pardus)
- Northern Hairy-nosed Wombat (Lasiorhinus krefftii)
- Numbat (Myrmecobius fasciatus)
- Orangutan (Pongo pygmaeus and Pongo abelii)
- Père David's Deer (Elaphurus davidianus)
- Proboscis Monkey (Nasalis larvatus)
- Red Panda (Ailurus fulgens)
- Red Wolf (Canis rufus)
- Savannah Elephant (Loxodonta africana)
- Sea Otter (Enhydra lutris)
- Sei Whale (Balaenoptera borealis)
- Snow Leopard (Uncia uncia)
- Steller's Sea Lion (Eumetopias jubatus)
- Sumatran Rhinoceros (Dicerorhinus sumatrensis)
- Temminck's Pangolin (Manis temminckii)
- Tibetan Antelope (Pantholops hodgsonii)
- Tiger (Panthera tigris)
- Vaquita (Phocoena sinus)
- Western Gorilla (Gorilla gorilla)
- West Indian Manatee (Trichechus manatus)

Endangered birds

- Alaotra Grebe (Tachybaptus rufolavatus)
- Amami Thrush (Zoothera major)
- Amsterdam Albatross (Diomedea amsterdamensis)
- Attwater's prairie-chicken (Tympanuchus cupido attwateri)
- Bald Eagle (Haliaeetus leucocephalus)
- Bali Starling (Leucospar rothschildi)
- Brazilian Merganser (Mergus octosetaceus)
- California Condor (Gymnogyps californianus)
- Chatham Albatross (Thalassarche eremita)
- Chinese Crested Tern (Sterna bernsteinii)
- Christmas Island Frigatebird (Fregata andrewsi)
- Cozumel Thrasher (Toxostoma guttatum)
- Eskimo Curlew (Numenius borealis)
- Giant Ibis (Thaumatibis gigantea)
- Glaucous Macaw (Anodorhynchus glaucus)
- Guam Rail (Gallirallus owstoni)
- Gurney's Pitta (Pitta gurneyi)
- Hawaiian Crow (Corvus hawaiiensis)
- Hawaiian Goose or Néné (Branta sandvicensis)
- Imperial Woodpecker (Campephilus imperialis)
- Indian White-rumped Vulture (Gyps bengalensis)
- Ivory-billed Woodpecker (Campephilus principalis)
- Jerdon's Courser (Rhinoptilus bitorquatus)
- Junin Flightless Grebe (Podiceps taczanowskii)
- Kakapo (Strigops habroptilus)
- Kittlitz's Murrelet (Brachyramphus brevirostris)
- Kiwi (Apteryx australis, A. hastii, A. owenii)
- Laysan Duck (Anas laysanensis)
- Lear's Macaw (Anodorhynchus leari)
- Lesser Sulphur-crested Cockatoo (Cacatua sulphurea)
- Long-billed Vulture, (Gyps indicus)
- Magdalena Tinamou (Crypturellus saltuarius)
- Magenta Petrel (Pterodroma magentae)
- Mauritius Fody (Foudia rubra)
- Mauritius Parakeet (Psittacula eques)
- Mindoro Bleeding-heart (Gallicolumba platenae)
- New Zealand Storm-petrel (Oceanites maorianus)
- Night Parrot (Geopsittacus occidentalis)
- Northern Bald Ibis (Geronticus eremita)
- Okinawa Woodpecker (Sapheopipo noguchii)
- Orange-bellied Parrot (Neophema chrysogaster)
- Philippine Eagle (Pithecophaga jefferyi)
- Po'ouli (Melamprosops phaeosoma)
- Puerto Rican Parrot (Amazona vittata)
- Raso Skylark (Alauda razae)
- Red-cockaded Woodpecker (Picoides borealis)
- Red-vented Cockatoo (Cacatua haematuropygia)
- Ridgway's Hawk (Buteo ridgwayi)
- São Tomé Grosbeak (Neospiza concolor)
- Siberian Crane (Grus leucogeranus)
- Slender-billed Curlew (Numenius tenuirostris)
- Socorro Mockingbird, (Mimodes graysoni)
- Sociable Lapwing (Vanellus gregarius)
- Spix's Macaw (Cyanopsitta spixii)
- White-headed Duck (Oxyura leucocephala)
- White-shouldered Ibis (Pseudibis davisoni)
- Whooping Crane (Grus americana)
- Writhed-billed Hornbill (Aceros waldeni)
- Zino's Petrel (Pterodroma madeira)
- Rama (Hyper Masochist)

Endangered reptiles

- St. Croix ground lizard (Ameiva polops)
- Loggerhead Sea Turtle (Caretta caretta)
- Green Sea Turtle (Chelonia mydas)
- Cuban Crocodile (Crocodylus rhombifer)
- Mona ground Iguana (Cyclura stejnegeri)
- Mesoamerican River Turtle (Dermatemys mawii)
- Leatherback Sea Turtle (Dermochelys coriacea)
- Hawksbill Sea Turtle (Eretmochelys imbricata)
- Blunt-nosed Leopard Lizard (Gambelia silus)
- Burmese Star Tortoise (Geochelone platynota)
- Kemp's Ridley Turtle (Lepidochelys kempii)
- Olive Ridley (Lepidochelys olivacea)
- Flat Back Turtle (Natator depressa)
- Grand Skink (Oligosoma grande)
- Otago Skink (Oligosoma otagense)
- Monito Gecko (Sphaerodactylus micropithecus)
- Coachella Valley fringe-toed lizard (Uma inornata)
- Komodo Dragon (Varanus komodoensis)
- Island Night Lizard (Xantusia riversiana)
- Horned Lizard (Phrynosoma)
- Wilson Ferreira (Totalis Negrus)

Endangered amphibians

- Arroyo Southwestern toad (Bufo californicus (=microscaphus))
- Barton Springs salamander (Eurycea sosorum)
- California tiger salamander (Ambystoma californiense)
- Desert slender salamander (Batrachoseps aridus)
- Fleishman's Glass Frog
- Houston toad (Bufo houstonensis)´
- Italian spade-footed toad (Pelobates fuscus insubricus)
- Mississippi gopher frog ( Rana capito sevosa)
- Mountain yellow-legged frog (Rana muscosa)
- Palmate newt (Triturus helvetica)
- Santa Cruz long-toed salamander (Ambystoma macrodactylum croceum)
- Shenandoah salamander (Plethodon shenandoah)
- Sonoran tiger salamander (Ambystoma tigrinum stebbinsi)
- Texas blind salamander (Eurycea rathbuni)
- Wyoming toad ( Bufo baxteri (=hemiophrys))
- Telmatobius (Telmatobius Wiegmann)

Endangered fish

- Gambusia eurystoma, native to Mexico, due to very limited habitat
- Nassau grouper (Epinephelus striatus)
- Chinese paddlefish (Psephurus gladius)
- Dwarf Pygmy Goby (Pandaka pygmae)
- Moapa dace
Endangered crustaceans

- Kentucky cave shrimp (Palaemonias ganteri)
- Alabama cave shrimp (Palaemonias alabamae)
Endangered arthropods

- Spruce-fir moss spider (Microhexura montivaga)
- Tooth cave spider (Neoleptoneta myopica)
Endangered mollusks

- Iowa Pleistocene Snail (Discus macclintocki)
Endangered plants
About 6% of the 300,000 identified species are endangered due to overcollection or destruction of habitat, among other causes. Pollinator decline is also a factor for some species.
- African violet (Saintpaulia ionantha), due to forest clearance
- Baishanzu fir (Abies beshanzuensis) of southeast China, three trees known on an isolated mountain summit
- Baker's larkspur (Delphinium bakeri) of California, due to very limited habitat
- Chilean wine palm (Juba chilensis), due to land clearance
- Dawn Redwood ("Metasequoia glyptostroboides"), thought to be extinct until 1941, when a small stand was discovered in China
- King of the Paphs Orchid (Paphiopedilum rothschildianum) of Asia, due to overcollection
- Lobster claw (Clianthus puniceus) of Australia, due to overgrazing
- Louisiana Quillwort, (Isoetes louisianensis) of Louisiana, due to very limited habitat
- Madonna lily (Lilium candidum) of Europe, due to overcollection
- Pinus squamata of southwest China, about 20 trees known
- Saguaro cactus (Carnegia gigantea) of North America, due to overcollection, slow maturing, and slow breeding
- Saharan Cypress (Cupressus dupreziana) of North Africa, due to small population and desertification
- Wollemi Pine (Wollemia nobilis) of Australia, also known as the 'Dinosaur Tree' or 'Living Fossil'. Wollemia fossils have been found in Australia, Antarctica and New Zealand. The plant was thought to be extinct until two trees were discovered in 1994. Research into the horticultural development of the Wollemi pine is being conducted at Mount Annan Botanic Garden and, with commercial propagation well under way, plants should be available in 2005.
See also

- Conservation status
- Convention on Biological Diversity
- Convention on the International Trade in Endangered Species of Wild Flora and Fauna
- Extinct birds
- Endangered Species Act
- International Convention for the Regulation of Whaling
- List of extinct and endangered animals of the British Isles
External links

- [http://news.bbc.co.uk/2/hi/science/nature/4013719.stm "Science counts species on brink"]. (Nov 17, 2004). BBC News.
- [http://www.sinapu.org/ Endangered Native Carnivores in the Southern Rockies]
- U.S. Fish & Wildlife Service. [http://ecos.fws.gov/tess_public/TESSWebpage Threatened and Endangered Species System (TESS)].
- [http://www.bagheera.com/index.cfm Bagheera]. A website for our endangered species. Category:Conservation ko:멸종위기종 ja:絶滅危惧種

Subway

:This article is about high-capacity urban rail public transit systems. For lower-order systems, see tram, light rail, bus, and bus rapid transit. :Metro and Subway redirect here. For other uses of the words, see metro (disambiguation) and subway (disambiguation). For specific rapid-transit systems, many of which use one of the two words in their names, see list of rapid transit systems. list of rapid transit systems A rapid transit, underground, subway, elevated, or metro system is a railway system, usually in an urban area, that usually has high capacity and frequency, with large trains and total or near total grade separation from other traffic.
Characteristics and nomenclature
There is no single term in English that all speakers would use for all rapid transit or metro systems. This fact reflects variations not only in national and regional usage, but in what characteristics are considered essential. One definition of a metro system is as follows:
- an urban, electric mass transit railway system
- totally independent from other traffic
- with high service frequency. But those who prefer the American term "subway" or the British "underground" would additionally specify that the tracks and stations must be
- located below street level so that pedestrians and road users see the street exactly as it would be without the subway; or at least that this must be true for the most important, central parts of the system. Conversely, those who prefer the American "rapid transit" or the newer (when used in English) term "metro" tend to view this as a less important characteristic and are pleased to include systems that are entirely elevated or at ground level (at grade) as long as the other criteria are met. A rapid transit system that is generally above street level may be called an "elevated" system (often shortened to el or, in Chicago, 'L'). In some cities the word "subway" applies to the entire system, in others only to those parts that actually are underground; and analogously for "el". For a more comprehensive listing showing names of this kind of system in cities around the world, see the list of rapid transit systems. Germanic languages generally use names meaning "underground railway" (such as "subway" or "U-Bahn"), while many others use "metro". See also passenger rail terminology.
Extent
passenger rail terminology In larger metropolitan areas the metro system may extend only to the limits of the central city, or to its inner ring of suburbs, with trains making relatively frequent station stops. The outer suburbs may then be reached by a separate commuter, suburban, or regional rail network, where more widely spaced stations allow a higher speed. These trains are often more expensive and less frequent, sometimes operating only in rush hours, and sometimes for political reasons they are operated by a separate authority that tends not to cooperate with the city's transit authority.rush hour Many of these regional railways were first built to operate in one direction from a city-center terminus, but some have been extended across the city center, sometimes running in tunnels. By making multiple stops in the city, they can offer suburban passengers a choice of stations, and also provide useful transportation within the city. A notable example is the Paris RER system, where (in cooperation with the city's transit authority) several pairs of existing suburban lines running in opposite directions from the city have been extended in tunnel to join up and form new through routes across the city. They are provided with frequent service and, within the city, the same fares as the Métro are charged, providing an integrated network. In German-speaking countries, such a system is called an S-Bahn. In Italian-speaking countries such a system is called Linea S or Treno Suburbano. In some cases, such as the San Francisco Bay Area Rapid Transit (BART) and Washington Metrorail systems, the rapid transit system itself runs to the suburbs and effectively functions as a regional rail service as well. Where there are separate systems, the rapid transit system is typically a self-contained service with its own dedicated tracks and stations and technologically incompatible with other railways. Suburban rail services, on the other hand, often share tracks and stations with long-distance trains (historically they were usually operated by the same company, which also owned the rails and ran freight, although this has become less common) and are subject to the same standards and regulations. There are exceptions; some London Underground lines share track with suburban rail services. In some cases, metro lines have been extended by taking over existing regional rail lines, notably the Central and Northern Lines in London. London Underground, Greece double as museums. Antiquities found in excavations of its tunnels are on display there.]]The Athens Metro's Blue Line shares tracks with suburban rail services in order to connect the metro to Eleftherios Venizelos International Airport, but does not stop at the suburban rail stations because the platforms of the stations are a lot lower than the train's floor. In Hong Kong, metro-like frequent service on the KCR East Rail to the new towns is provided by electrifying existing railway line, while continues to share part of the tracks with the much less frequent intercity and freight trains. The KCR West Rail, on the contrary, is designed to accommodate intercity and freight traffic in future, while presently provides only metro-like service. Elevated railways were a popular way to build mass transit systems in cities around the turn of the twentieth century, but they have fallen out of favour and many elevated lines were later demolished, being replaced by subways or buses. Elevated rail saw something of a resurgence in the late twentieth century, with the construction of a number of new lines such as the Docklands Light Railway in London and the Vancouver SkyTrain; in the United States a few such lines have been built, including the AirTrain JFK and the Las Vegas Monorail, but these are typically seen as more futuristic, and are not representative of the overall trends in U.S. transit development.
Importance, functions, and station design
The volume of passengers a metro train can carry is often quite high, and a metro system is often viewed as the backbone of a large city's public transportation system. In many cities passengers beginning their journeys on a streetcar/tram, bus, or suburban rail system must finish their journey into the city center on the metro as their first mode of transport will terminate at a metro station to avoid congesting the city center above ground. Budapest is a perfect example where the two more modern metro lines connect with buses and trams and also with two circular streetcar/tram routes (one closer to and one further from the city center) that allow travel between suburbs and also into the centre of the city by changing onto the metro. In some cities, the urban rail system is so comprehensive and efficient that the majority of city residents go without an automobile. Hong Kong, London, Moscow, New York City, Madrid, Paris, and Tokyo have the most extensive and advanced metro systems in the world. Chicago, Washington, D.C., and Boston follow New York distantly, while the rest of the cities in the United States only have partial or poorly-used systems, such as Los Angeles, Saint Louis or Philadelphia. In the Western Hemisphere, Mexico City also has a large system. In Canada, only Toronto and Montréal have extensive metro networks serving their urban centers (see Toronto subway and RT and Montréal Métro); Vancouver's SkyTrain also provides high-grade service, but at present acts primarily as a connection between Vancouver and the surrounding area. SkyTrain Most underground systems are for public transportation, but a few cities have built freight or postal lines. One example was the Post Office Railway, which transported mail underground between sorting offices in London from 1927 until it was abandoned in 2003. Similarly, in its early days the London Underground's Metropolitan Line (then the Metropolitan Railway) transported goods as well as running passenger trains. Another example was the Chicago Tunnel Company, which had a dense grid of tunnels under downtown Chicago. During the Cold War an important secondary function of some underground systems was to provide shelter in case of a nuclear attack. Urban rail systems have often been used to showcase economic, social, and technological achievements of a nation, especially in the Soviet Union and other Communist countries. With their marble walls, polished granite floors and splendid mosaics, the metro systems of Moscow and St. Petersburg are widely regarded as some of the most beautiful in the world. Modern metro stations in Russia are usually still built with the same emphasis on appearance. Similarly, the Independent Subway System in New York City was built to compete with the private IRT and BMT systems, and succeeded in running them out of business (in conjunction with other factors, such as fare limits). The New York City subway system (actually only approximately 60% of total track miles are underground) is now owned by the city government. In fact, almost all subway systems around the world are owned by government entities. BMT.]] See also Metro station.
Technology

Train size and motive power
Some urban rail lines are built to the full size of main-line railways; others use smaller tunnels, restricting the size and sometimes the shape of the trains (in the London Underground the informal term tube train is commonly used). Some lines use light rail rolling stock, perhaps surface cars simply routed into a tunnel for all or part of their route. In many cities, such as London and Boston's MBTA, lines using different types of vehicles are organised into a single unified system (though often not connected by track). Although the initial lines of what became the London Underground used steam engines, most metro trains, both now and historically, are electric multiple units, with steel wheels running on two steel rails. Power for the trains, referred to as traction power, is commonly supplied by means of a single live third rail (as in New York) at 600 to 750 volts, but some systems use two live rails (notably London) and thus eliminate the return current from the running rails. Overhead wires, allowing higher voltages, are more likely to be used on metro systems without much length in tunnel, as in Amsterdam; but they also occur on some that are underground, as in Madrid. Boston's Green Line trains derive power from an overhead wire, both while traveling in a tunnel in the central city and at street level in the suburban areas. Systems generally use DC power rather than AC, even though this requires large rectifiers for the power supply. DC motors were formerly more efficient for railway applications, and once a DC system is in place, converting it to AC is generally considered too large a project to contemplate.
Tracks
motor Most rapid transit systems use conventional railway tracks, although since tracks in subway tunnels are not exposed to wet weather, they are often fixed to the floor rather than resting on ballast. The rapid transit system in San Diego, California operates tracks on former railroad rights of way that were acquired by the governing entity. An alternative technology using rubber tires on narrow concrete or steel rollways was pioneered on the Paris Métro, and the first complete system to use it was in Montréal. Additional horizontal wheels are required for guidance, and a conventional track is often provided in case of flat tires and for switching. Advocates of this system note that it is much quieter than conventional steel-wheeled trains, and allows for greater inclines given the increased traction allowed by the rubber tires. Some cities with steep hills incorporate mountain railway technologies into their metros. The Lyon Metro includes a section of rack (cog) railway, while the Carmelit in Haifa is an underground funicular. For elevated lines, still another alternative is the monorail. Supported or "straddle" monorails, with a single rail below the train, include the Tokyo Monorail; the Schwebebahn in Wuppertal is a suspended monorail, where the train body hangs below the wheels and rail. Monorails have never gained wide acceptance outside Japan, though Seattle has a short one (in November 2005 voters in Seattle decided against expanding this system, which dates to the World's Fair of 1962), and one has recently been built in Las Vegas. One of the first monorail systems in the United States was installed at Anaheim's Disneyland in 1959 and connects the amusement park to a nearby hotel. Disneyland's builder, animator and filmmaker Walt Disney, offered to build a similar system between Anaheim and Los Angeles.
Crew size and automation
Los Angeles trains, manufactured by Alstom of France, are fully automated and are not manned by any driver.]] Early underground trains often carried an attendant on each car to operate the doors or gates, as well as a driver (often called the "motorman"). The introduction of powered doors around 1920 permitted crew sizes to be reduced, and trains in many cities are now operated by a single person. Where the operator would not be able to see the whole side of the train to tell whether the doors can be safely closed, mirrors or closed-circuit TV monitors are often provided for that purpose. closed-circuit TV An alternative to human drivers became available in the 1960s, as automated systems were developed that could start a train, accelerate to the correct speed, and stop automatically at the next station, also taking into account the information that a human driver would obtain from lineside or cab signals. The first complete line to use this technology was London's Victoria Line, in 1968. In normal operation the one crew member sits in the driver's position at the front, but just closes the doors at each station; the train then starts automatically. This style of system has become widespread. A variant is seen on London's Docklands Light Railway, opened in 1987, where the "passenger service agent" (formerly "train captain") rides with the passengers rather than sitting at the front as a driver would. The same technology would have allowed trains to operate completely automatically with no crew, just as most elevators do; and as the cost of automation has decreased, this has become financially attractive. But a countervailing argument is that of possible emergency situations. A crew member on board the train may be able to prevent the emergency in the first place, drive a partially failed train to the next station, assist with an evacuation if needed, or call for the correct emergency services (police, fire, or ambulance) and help direct them. emergency.]] In some cities the same reasons are considered to justify a crew of two rather than one; one person drives from the front of the train, while the other operates the doors from a position farther back, and is more conveniently able to assist passengers in the rear cars. The crew members may exchange roles on the reverse trip (as in Toronto) or not (as in New York). Completely unmanned trains are more accepted on newer systems where there are no existing crews to be removed, and especially on light rail lines. Thus the first such system was the VAL (véhicule automatique léger or "automated light vehicle") of Lille, France, inaugurated in 1983. Additional VAL lines have been built in other cities. In Canada, the Vancouver SkyTrain carries no crew members, while Toronto's Scarborough RT, opening the same year (1985) with otherwise identical trains, uses human operators. These systems commonly use platform-edge doors (PEDs), in order to improve safety and ensure passenger confidence, but this is not universal: for example, the Vancouver SkyTrain does not. (And conversely, some lines which retain drivers nevertheless use PEDs, notably London's Jubilee Line Extension. MTR of Hong Kong also uses platform screen doors, the first to install PSDs on an already operating system.) Rapid transit systems in the United States do not use PEDs, with the exception of the Las Vegas Monorail which was the first system to use them in the country because of the city's desert climate. As to larger trains, the Paris Metro has human drivers on most lines, but runs crewless trains on its newest line, Line 14, which opened in 1998. Singapore's North East MRT Line (2003) claims to be the world's first fully automated underground urban heavy rail line. The Disneyland Resort Line of Hong Kong MTR is also automated, with a staff riding with the passengers. :See also People mover.
Tunnel construction
People mover] The construction of an underground metro is an expensive project, often carried out over a number of years. There are several different methods of building underground lines. In one common method, known as cut-and-cover (used in the first New York City subway line), the city streets are excavated and a tunnel structure strong enough to support the road above is built at the trench, which is then filled in and the roadway rebuilt. This method (used for most of the underground parts of the São Paulo and Guadalajara subways, for example) often involves extensive relocation of the utilities commonly buried not far below city streets – particularly power and telephone wiring, water and gas mains, and sewers. This relocation must be done carefully, as according to documentaries from the National Geographic Society, one of the causes of the April 22 explosions in Guadalajara, which happened in 1992, was a misrelocated water pipeline. The structures are typically made of concrete, perhaps with structural columns of steel; in the oldest systems, brick and cast iron were used. Cut-and-cover construction can take so long that it is often necessary to build a temporary roadbed while construction is going on underneath in order to avoid closing main streets for long periods of time; in Toronto, a temporary surface on Yonge Street supported cars and streetcar tracks for several years while the Yonge subway was built. Some American cities, like Newark, Cincinnati and Rochester, were initially built around canals. When the railways replaced canals, they were able to bury a subway in the disused canal's trench, without rerouting other utilities, or acquiring a right of way piecemeal. Another usual way is to start with a vertical shaft and then dig the tunnels horizontally from there, often with a tunnelling shield, thus avoiding almost any disturbance to existing streets, buildings, and utilities. But problems with ground water are more likely, and tunnelling through native bedrock may require blasting. (The first city to extensively use deep tunneling was London, where a thick sedimentary layer of clay largely avoids both problems.) The confined space in the tunnel also limits the machinery that can be used, but specialised tunnel-boring machines are now available to overcome this challenge. One disadvantage with this, however, is that the cost of tunnelling is much higher than building systems cut-and-cover, at-grade or elevated. Early tunnelling machines could not make tunnels large enough for conventional railway equipment, necessitating special low, round trains, such as are still used by most of the London Underground, which cannot install air conditioning on most of its lines because the amount of empty space between the trains and tunnel walls is so small. The deepest metro system in the world was built in St. Petersburg, Russia. In this city, built in the marshland, stable soil starts more than 50 metres deep. Above that level the soil mostly consists of water-bearing finely dispersed sand. Because of this, only three stations out of nearly 60 are built near the ground level and three more above the ground. Some stations and tunnels lie as deep as 100-120 meters below the surface. However, the location of the world's deepest station is not as clear. Usually, the vertical distance between the ground level and the rail is used to represent the depth. Among the possible candidates are: marshland, Russia metro depicts Ancient Greece; the word "sportivnaya" means "sporty" or "athletic".]]
- Deepest stations in St. Petersburg, Russia:
- Admiraltejskaya (The Admiralty, 102 meters, still in construction, probably the best candidate)
- Komendantskij Prospekt (The Commandant Avenue, 78 meters, launched 2005)
- Chernishevskaya (Chernyshevsky, 74 meters, launched 1958)
- Ploshad Lenina (Lenin Square, 72 meters, launched 1958)
- Arsenal'na station in Kyiv, Ukraine (built under a hill)
- Park Pobedy station in Moscow Metro (built under a hill)
- Puhung station in Pyongyang, North Korea (the Pyongyang metro doubles as a nuclear shelter)
- Washington Park station on Metropolitan Area Express in Portland, Oregon (built under a hill), 260 feet (80 m) One advantage of deep tunnels is that they can dip in a basin-like profile between stations, without incurring significant extra costs due to having to dig deeper. This technique, also referred to as putting stations "on humps", allows gravity to assist the trains as they accelerate from one station and brake at the next. It was used as early as 1890 on parts of the City and South London Railway, and has been used many times since. The proposed West Island extention to the Island Line of the MTR of Hong Kong will have stations over 100 metres below the ground level, to serve passengers on the Mid-levels. According to the latest proposal some of the entrances/exits will be equipped with high-speed lifts, instead of the conventional way to use escalators.
History
Mid-levels.]] The 2750-foot (850-m) Cobble Hill Tunnel in Brooklyn, New York (now part of New York City) is claimed to be the "world's oldest subway tunnel". This was formed in 1850 when an open cut on the Brooklyn and Jamaica Railroad in the middle of Atlantic Avenue was bricked over to form a tunnel. The company was later taken over by the Long Island Rail Road, but the tunnel was closed in 1861. This tunnel was not a true subway, as it had no stations and was used for long-distance regional rail and streetcars. Soon after, the similar Murray Hill Tunnel on the New York and Harlem Railroad was roofed over in the 1850s. This ran under Park Avenue i