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This page refers to urban rail mass transit systems. For other uses see metro (disambiguation).

Underground, subway and metro are common names for a form of mass transit public transport system employing trains. In many cases, at least a portion of the rails are placed in tunnels dug beneath the surface of a city. [1]

A person with a devoted interest in these systems is a metrophile.

Table of contents
1 Definition and nomenclature
2 Importance and functions
3 Metro trains
4 Drivers and automation
5 Subway construction
6 History
7 See also
8 External links

Definition and nomenclature

One definition of a "true" metro system is as follows:

  1. an urban, electric mass transit system
  2. totally independent from other traffic
  3. with high service frequency.

Those who prefer the American term "subway" or the British "underground" would additionally specify that at least the most important, central parts of the system must be located below street level; those who prefer "metro" tend to view this as a less important characteristic and are pleased to include systems that are entirely elevated or at grade. In some cities "subway" refers to the entire system, in others only to the portions that actually are underground.

For a more comprehensive listing of other names of this kind of system in cities around the world, see the list of metro systems. Germanic languages generally use names meaning "underground railway", while many others use "metro".

Importance and functions

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.

Most underground systems are for public transportation, but a few cities have built freight (Chicago Freight Subway) or postal lines. One example was the Post Office Railway, which transported mail underground between sorting offices in London from 1927 until it was "mothballed" in 2003. During the Cold War an important secondary function of some underground systems was to provide shelter in case of a nuclear attack.

Metro systems have often been used to showcase economical, social, and technological achievements of a nation, especially in the Soviet Union and other socialist 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.

Metro trains

Some metro systems 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 London the informal term tube train is commonly used), or use light rail rolling stock. The trains usually stop at frequent intervals to let passengers on or off.

Drivers and automation

Traditionally, metro trains are driven by human drivers, but automated trains also exist, in, for example, London (the Victoria Line), Singapore, and Paris. This is not a recent invention; operation of trains on the Victoria Line has been automatic since its opening in 1968. However, in common with most systems, an operator is still carried in a cab at the front of the train. The VAL (véhicule automatique léger) of Lille, inaugurated in 1983, provided the first driverless underground system. Other driverless lines now include the line 14 (Meteor) of the Paris Metro, opened in 1998. The Docklands Light Railway (1987) in London, whilst for the most part not underground, is also driverless. Singapore's North-East Line (2003) claims to be the world's first fully automated heavy rail line. See also People mover.

Subway construction

The construction of an underground is an expensive project, often carried out over a number of years. Several modes of tunneling exist. In one common method, known as cut-and-cover, 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 Săo Paulo metro, 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. The structures are typically made of concrete, perhaps with structural columns of steel; in the oldest systems, brick and cast iron were used.

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 specialized 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.

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 meters deep. Above that level the soil moslty consists of water-bearing finely dispersed sand. Because of this, only 3 stations out of nearly 60 are built near the ground level and 3 more above the ground. Many stations lie as deep as 100 meters below the surface. However, the location of the world deepest station is not as clear. Among the possible candidates are:

One advantage of deep tunnels is that they can dip in a basin-like profile between stations, without incurrent 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.

Underground systems use a variety of technologies. Most systems run on steel wheels and rails, although a number of modern systems use rubber tires and concrete rollways. (The Montreal metro was the first completely rubber-tired metro system.) Power is usually supplied either by means of a single third rail (New York), but some systems use two live rails (London) or overhead lines (Madrid). Systems may be underground, at grade, elevated, or a mix as in the Paris metro. Some systems use light rail; other cities' systems are hybrids wherein a tramway moves underground in the city centre.

Due to the complexity of construction and operations, underground systems need constant investment from the public authority, to avoid disasters like King's Cross fire in London's Underground.


In 1844 the Long Island Rail Road opened the Atlantic Avenue Tunnel, carrying its line for about 500 m under the streets of Brooklyn (now part of New York City). Although sometimes called the "world's oldest subway tunnel", this had no stations and was used for long-distance as well as suburban trains.

The first real underground line in the sense discussed here was the Metropolitan Railway in London, which opened in 1863, using condensing steam locomotives as no better motive power was available. It was an immediate success and many extensions followed; the Metropolitan eventually became an important part of the London Underground system. Steam working underground lasted until 1905.

In 1870 short single-track lines opened in both New York and London using alternative technologies. In New York, Alfred Beach built a 95 m tunnel (with a single station and a dead end at the other end) to demonstrate pneumatic train operation; this operated until 1873, after which the tunnel became a rifle range and was then abandoned. In London, the Tower Subway provided a crossing under the River Thames using a tiny cable car for the 410 m journey; the line closed in a matter of months and the tunnel was given over to pedestrians, later becoming a utility conduit.

The first deep-level underground line was the City and South London Railway, which opened in 1890. Steam operation being considered ridiculous, cable traction was chosen; but during construction the management decided to try electric locomotives instead, and so the C&SLR became the first underground electric railway and the first important electric railway of any kind.

The first underground railway in continental Europe was completed in Budapest, Hungary in 1896, after only two years of construction between Vörösmarty tér in the city centre and Széchenyi fürdő as the first electric underground (Földalatti) line on the European mainland.

The first line of the Paris Metro opened in 1900. Its full name was the Chemin de Fer Métropolitain, a direct translation of London's Metropolitan Railway. The name was shortened to metro in French, and this word was borrowed by many other languages.

Boston has the oldest subway system in the United States, the green line. Later lines carried full-size trains rather than trams. The first section of the New York Subway, which became the world's largest (by some measures), did not open until 1904.

The oldest underground in Latin America was built in 1913 in Buenos Aires.

The Toronto subway (1953) was the first to replace street transit routes. Toronto also developed the aluminum subway car, which reduced operating costs.

In Brazil, the first underground opened in 1974 in Săo Paulo, and and now carries some 4 million passengers on an average week day. Part of it consists of converted older railways; some of its stations actually date from the 1880's. Underground lines have been built also in Rio de Janeiro, Belo Horizonte, Recife, Porto Alegre and Brasília.

Asia's oldest subway line is Tokyo's Ginza Line, opened in 1927.

See also

External links