A rapid transit, subway, underground, elevated railway or metro(politan) system is an electric passenger railway in an urban area with high capacity and frequency, and which is grade separated from other traffic. Rapid transit systems are typically either in tunnels or elevated above street level. Outside urban centres rapid transit lines sometimes run grade separated at ground level.
Service on rapid transit systems is provided on designated lines between stations using electric multiple units on rails, although some systems use magnetic levitation or monorails. They are typically integrated with other public transport and often operated by the same public transit authorities. Rapid transit is faster and has a higher capacity than trams or light rail, 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. Since then the largest growth has been in Asia and with driverless systems. More than 160 cities have rapid transit systems, totalling more than 8,000 km (4,900 miles) of track and 7,000 stations. Twenty-five cities have systems under construction.
