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.