Subway. Rapid transit or mass rapid transit, also known as heavy rail or metro, is a type of high-capacity public transport that is generally built in urban areas.
A rapid transit system that primarily or traditionally runs below the surface may be called a subway, tube, or underground. Unlike buses or trams, rapid transit systems are railways, usually electric, that operate on an exclusive right-of-way, which cannot be accessed by pedestrians or other vehicles.
They are often grade-separated in tunnels or on elevated railways. Modern services on rapid transit systems are provided on designated lines between stations typically using electric multiple units on railway tracks.
Some systems use guided rubber tires, magnetic levitation, or monorail. The stations typically have high platforms, without steps inside the trains, requiring custom-made trains in order to minimize gaps between train and platform.
They are typically integrated with other public transport and often operated by the same public transport authorities. Some rapid transit systems have at-grade intersections between a rapid transit line and a road or between two rapid transit lines. The world's first rapid transit system was the partially underground Metropolitan Railway which opened in 1863 using steam locomotives, and now forms part of the London Underground. In 1868, New York opened the elevated West Side and Yonkers