A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on elevated rails above street level. Outside urban centers, rapid transit lines may run on grade separated ground level tracks. The first rapid transit system was the London Underground, which opened in The technology quickly spread to other cities in Europe, and then to the United States where a number of elevated systems were built. At first these systems used steam locomotives, with the term later coming to entirely mean electric systems. Since then the largest growth has been in Asia and with driverless systems. More than 160 cities have rapid transit systems, totaling more than 8,000 km (5,000 mi) of track and 7,000 stations. Twenty-five cities have new systems under construction. Metro is the most common term for underground rapid transit systems. Rapid transit systems may be named after the medium through which their busier inner-city sections travel: use of tunnels inspires names such as subway, underground, Untergrundbahn (U-bahn) in German,or Tunnelbana (T-bana) in Swedish; use of viaducts inspires names such as elevated, skytrain, overhead or overground. One of these terms may apply to an entire system, even if a large part of the network (for example, in outer suburbs) runs on ground level. In English English a subway is a pedestrian underpass, so the expressions underground and tube may be preferred. In Scotland, Glasgow's underground rapid transit system is called the Glasgow Subway.

Rapid transit evolved from steam railways during the late 19th century. In 1890 the City & South London Railway in London was the first electric rapid transit railway. The electric railway eventually was merged into London Underground. The technology swiftly spread to other cities in Europe, as in Budapest, Hungary in 1896, and then to the United States. A number of elevated systems were built, starting with the % designed electric Liverpool Overhead Railway. The elevated railways in Chicago and New York were converted to electric from steam propulsion. By 1940, there were 19 systems, and by 1984, there were 66. This included smaller cities like Oslo and Marseille which opened extensive systems in the 1960s. More recently the growth of new systems has been concentrated in Southeast Asia and Latin America. Western Europe and North America have instead seen a revival of the tram, with light rail systems supplementing full scale urban railways, and less focus on building rapid transit. At the same time, technological improvements have allowed new driverless lines and systems. Hybrid solutions have also evolved, such as tram- train and premetro, which have some of the features of rapid transit systems.

Particularly in the former Soviet Union and other Eastern European countries, but to an increasing extent elsewhere, the stations were built with splendid decorations such as marble walls, polished granite floors and mosaicsthus exposing the public to art in their everyday life, outside galleries and museums. The systems in Moscow and St.Petersburg are widely regarded as some of the most beautiful in the world, but several other cities such as Stockholm, Montreal, Lisbon, and Los Angeles have also focused on art, which may range from decorative wall claddings, to large, flamboyant artistic schemes integrated with station architecture, to displays of ancient artifacts recovered during station construction.It may be possible to profit by attracting more passengers by spending relatively small amounts on grand architecture, art, cleanliness, accessibility, lighting and a feeling of safety.Particularly in the former Soviet Union and other Eastern European countries, but to an increasing extent elsewhere, the stations were built with splendid decorations such as marble walls, polished granite floors and mosaicsthus exposing the public to art in their everyday life, outside galleries and museums. The systems in Moscow and St.Petersburg are widely regarded as some of the most beautiful in the world, but several other cities such as Stockholm, Montreal, Lisbon, and Los Angeles have also focused on art, which may range from decorative wall claddings, to large, flamboyant artistic schemes integrated with station architecture, to displays of ancient artifacts recovered during station construction.It may be possible to profit by attracting more passengers by spending relatively small amounts on grand architecture, art, cleanliness, accessibility, lighting and a feeling of safety.

By length of lines the largest are the Shanghai Metro and London Underground. Shanghai MetroLondon UndergroundShanghai MetroLondon Underground

The biggest metro system in the world by length of routes and number of stations is the New York Subway New York SubwayNew York Subway

Tokyo subway Moscow Metro Seoul Metropolitan Subway The busiest metro systems in the world by daily and annual ridership are the Tokyo subway, Moscow Metro and Seoul Metropolitan Subway.Tokyo subwayMoscow Metro Seoul Metropolitan SubwayTokyo subwayMoscow Metro Seoul Metropolitan Subway

The first plans for a metro system in Moscow date back to the Russian Empire but were postponed by World War I, the October Revolution and the Russian Civil War. In 1923, the Moscow City Council formed the Underground Railway Design Office at the Moscow Board of Urban Railways. It carried out preliminary studies, and by 1928 had developed a project for the first route from Sokolniki to the city centre. At the same time, an offer was made to German company Siemens Bauunion to submit its own project for the same route. In June 1931, the decision to begin construction of the Moscow Metro was made by the Central Committee of the Communist Party of the Soviet Union. In January 1932 the plan for the first lines was approved, and on March 21, 1933 the Soviet government approved a plan for 10 lines with a total route length of 80 km. The first lines were built using the Moscow general plan designed by Lazar Kaganovich in the 1930s, and the Metro was named after him until 1955 named after him (Metropoliten im. L.M. Kaganovicha). The Moscow Metro construction engineers consulted with their counterparts from the London Underground, the world's oldest metro system. Partly because of this connection, the design of Gants Hill tube station (although not completed until much later) is reminiscent of a Moscow Metro Station.

The Moscow Metro encompasses 182 stations, of which 73 are deep below ground and 88 shallower. Of the deep stations 52 are pylon-type, 18 are column-type and one is "single-vault" (Leningrad technology). The shallow stations comprise 63 pillar-type (a large portion of them following the "centipede" design), 20 single-vaults (Kharkov technology) and three single-decked. In addition, there are 11 ground-level stations and four above ground. Two of the stations exist as double halls, and two have three tracks. Five of the stations have side platforms (only one subterranean; that station Vorobyovy Gory is on a bridge). Three other metro bridges exist, but are covered or hidden. In addition, there are two closed stations and one that is in disrepair. Four stations are reserved for future service: Volokolamskaya on the Tagansko-Krasnopresnenskaya line, Delovoy Tsentr stations on the Kalininskaya and Solntsevskaya lines and Park Pobedy on the Solntsevskaya line.

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