Rapid transit

Rapid transit or mass rapid transit (MRT) or heavy rail, 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, or underground. They are sometimes grade-separated on elevated railways, in which case some are referred to as el trains – short for "elevated" – or skytrains. Rapid transit systems are railways, usually electric, that unlike buses or trams operate on an exclusive right-of-way, which cannot be accessed by pedestrians or other vehicles.