Rapid transit, mass rapid transit (MRT) or rail rapid transit (RRT) and commonly referred to as metro, is a type of high-capacity public transport that is generally built in urban areas. A grade separated rapid transit line below ground surface through a tunnel can be regionally called a subway, tube, metro or underground. They are sometimes grade-separated on elevated railways, in which case some are referred to as elevated, el or L trains – short for "elevated" – or skytrains. A common alternative term for rapid transit in North America is heavy rail. Rapid transit systems are usually electric railways that, unlike buses or trams, operate on an exclusive right-of-way, which cannot be accessed by pedestrians or other vehicles.
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 (maglev), 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.
