Rapid transit technology
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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.
Most rapid transit systems use conventional railway tracks and standard gauge, 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 Montreal. 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
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.
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.
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.
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 Métro 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) is 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 and Automatic platform gate.
- See also 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, 1992 explosions in Guadalajara 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 (160 ft) 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 metres (330–390 ft) 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:
- Deepest stations in Saint Petersburg Metro, Russia:
- Admiralteyskaya (The Admiralty, 102 metres (335 ft), launched 2011, probably the best candidate)
- Komendantsky Prospekt (The Commandant Avenue, 78 metres (256 ft), launched 2005)
- Chernyshevskaya (Chernyshevsky, 70 metres (230 ft), launched 1958)
- Ploshad Lenina (Lenin Square, 72 metres (236 ft), launched 1958)
- Arsenalna station in Kiev Metro, Ukraince (about 102 metres (335 ft), launched 1960, built under a hill)
- Park Pobedy station in Moscow Metro (~80 metres (260 ft), launched 2005, 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 extension to the Island Line of the MTR of Hong Kong will have stations over 100 metres (330 ft) 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.