A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is a passenger transport system 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 viaducts above street level. Outside urban centers, rapid transit lines may run on grade separated ground level tracks.
Service on rapid transit systems is provided on designated lines between stations using electric multiple units on rail tracks, although some systems use guided rubber tyres, magnetic levitation, or monorail. They are typically integrated with other public transport and often operated by the same public transport authorities. Rapid transit is faster and has a higher capacity than trams or light rail (but does not exclude a fully grade separated LRT), but is not as fast or as far-reaching as commuter rail. It is unchallenged in its ability to transport large amounts of people quickly over short distances with little land use. Variations of rapid transit include people movers, small-scale light metro and the commuter rail hybrid S-Bahn.
The first rapid transit system was the London Underground, which opened in 1863. 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 usedsteam locomotives, with the term later coming to entirely mean electric systems. More recently the largest growth has been in Asia and with driverless systems. More than 178 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.
The biggest rapid transit system in the world by length of routes (including non-revenue track) and by number of stations is the New York City Subway; by length of passenger lines, the largest are the Shanghai Metro and London Underground. Thebusiest metro systems in the world by daily and annual ridership are the Tokyo subway, the Seoul Metropolitan Subway, and the Moscow Metro.
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