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The underground, subway or metro are common names for a form of mass transit public transport system, employing small trains where at least a portion of the rails are placed in tunnels dug beneath the surface of a city.

A bewildering flora of names have grown up that all refer to this kind of transportation, but by virtue of being the first in operation, the London Underground has given its name to this article about the phenomenon itself. For a more comprehensive listing of other names of this kind of system, see Underground listing.

The underground trains usually stop at short intervals to let passengers on or off. The volume of passengers an underground train can carry is often quite high, and an underground system is often viewed as the backbone of a large city's public transportation system.

Traditionally, underground trains are driven by human drivers, but recently automated trains have been employed, for example in Singapore and London's Docklands Light Railway.

The construction of an underground is an expensive project, often carried out over a number of years. Several modes of tunneling exist. One common method is to place the tracks directly beneath the city streets, upholding the roads by concrete pillars (parts of the New York Subway system are constructed in this manner, known as Cut-and-cover). Another usual way is to dig the tunnels (often with Tunnelling shields) beneath previously occupied subterranean space, through native bedrock, and seal the tunnels from leakage of ground water with concrete.

Undergrounds are found in many cities around the world: here is a list.


The oldest subway tunnel in the world is the Atlantic Avenue Tunnel, built in 1844.

Boston has the the oldest subway system in the United States.

The underground is sometimes a nickname for a resistance movement: "the underground" is also a common name for World War II resistance movements. By extension, the term was also applied to counter-cultural movement(s) many of which sprang up during the 1960s.