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A Rapid transit system (also called a Metropolitan), 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 many passengers. Usually it lacks level crossings. Rather, grade separation keeps it separate from other traffic. Grade separation is when two streams of traffic go over and under each other, so they do not meet.
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.
The oldest rapid transit system is the London Underground. Building started in 1860 and the first part of it was opened in 1863. The first underground trains were powered by steam. Another part of the London Underground was the first rapid transit using electric trains, as is normal today. This part opened in 1890.