A Rapid transit system (also called a Metro(politan), subway, underground, tube, elevated, or heavy rail system) is a railway system. Usually it is in a city or urban area. Trains run very often, and carry a lot of passengers. Usually it is separated from other traffic by a concept called grade separation. Grade separation is when two streams of traffic cross each other on different levels, so they do not influence each other.
There are two major types of rapid transit—elevateds and subways. A single rapid transit system may have both elevated and subway lines.
Elevated rapid transit lines are cheaper to build than subways since no tunneling is required. A well-known mostly elevated rapid transit system is the Bay Area Rapid Transit system in the San Francisco Bay Area. Construction began in 1964 and the first trains began running in 1972. However, all of the lines in this system do travel through subways part of the way.
In England a subway is called an Underground, in Scotland and North America it is called Subway, in Germany and Austria U-Bahn and in the rest of the world usually Metro, for example: Barcelona Metro.
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