Rapid transit describes a type of urban rail transportation, generally including subway and elevated lines in the U.S., Metros in most other countries, and U-Bahnen in Germany.

Originally, the term rapid transit was used beginning in the 1800s to describe new forms of quick urban public transportation that separated the rapid transit right-of-way from street traffic. This set rapid transit apart from horsecars, trams, streetcars, omnibuses, and other forms of public transport.

Though the term was almost always used to describe rail transportation, other forms of transit were sometimes described by their proponents as rapid transit, even including local ferries in some cases.

In the 19th century, elevated steam railroads were the most common form of rapid transit, though steam powered trains were used in the London Underground tunnels in that era. Rapid transit locomotives were specially configured to start and stop quickly, the opposite of mainline railroad engines, in order to provide quick service where stations were located close together.

In the modern context, rapid transit is distinguished, with rare exceptions, by a number of characteristics:

  • Electrically powered conventional rail lines usually using third rail but sometimes overhead wire, for power collection;

  • Trains ranging from two to eleven cars using mutliple-unit operation, where each car has its own motors, all controlled by an operator in the lead car;

  • High-level platforms, where the platforms are the same height as the floors of the trains, for fast and safe loading;

  • Headway operation, where trains operate at fixed intervals on pre-determined routings, such as every 10 minutes, rather than on a timetable basis, like railroads;

  • Pre-paid fare collection, obviating the need for personnel to collect cash or ticket fares on-board;

  • Frequent stations in urban areas, sometimes as close as a quarter-mile (400m) apart on older systems, lengthening to as much as several miles (several km) in less dense areas, especialy on the newest systems.

Sites: rapidtransit.com, which includes links to operating companies, and New York City Subway Resources, an extensive site that includes many photos and much information about rapid transit systems in the U.S. and worldwide, in addition to New York City.