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.