A rapid transit, underground, subway, tube, 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 parts of the world these systems are known as a "metro", in London the system is called the "underground", while in most of North America and in Glasgow, Scotland it is a "subway".
The oldest rapid transit system in the world is the London Underground, which opened in 1863. The two primary ways that subway tunnels are constructed are by cut and cover and tunnel boring. The world's most extensive urban rail systems are Tokyo's rail network and Osaka, Japan.
162 cities have rapid transit systems, totalling more than 8,000 km (4,900 miles) of track and 7,000 stations. 25 cities have new systems under construction.citation needed]