A rapid transit, underground, subway, elevated or metro(politan) system is a railway — usually in an urban area — with a high capacity and frequency of service and grade separation from other traffic. In most of the world, these systems are known as a "Metro." In London the system is called the "Underground" but popularily known as 'the Tube', in Buenos Aires "Subte", (a short colloquial word for subterráneo, Spanish for underground), "Metrô" (short for Metropolitan) in São Paulo and Rio de Janeiro and in most of North America and in Glasgow, Scotland, "subway". The oldest rapid transit system in the world is the London Underground, which opened in 1863 and was then called the Metropolitan Railway. The Underground remains one of the most extensive rapid transit systems in the world.

The two primary ways that subway tunnels are constructed are by cut and cover and tunnel boring.

One hundred sixty-two cities have rapid transit systems, totaling more than 8,000 km (4,900 miles) of track and 7,000 stations. Twenty-five cities have new systems under construction.