This page refers to urban rail mass transit systems. For other uses see metro (disambiguation).
Underground, subway and metro are common names for a form of mass transit public transport system employing small trains, these are also called in Britain tube trains. In many cases, at least a portion of the rails are placed in tunnels dug beneath the surface of a city. 
One definition of a "true" metro system is as follows:
- an urban, electric mass transit system
- totally independent from other traffic
- with high service frequency.
For a more comprehensive listing of other names of this kind of system in cities around the world, see the list of metro systems.
The metro trains usually stop at short intervals to let passengers on or off. The volume of passengers a metro train can carry is often quite high, and a metro system is often viewed as the backbone of a large city's public transportation system.
Traditionally, metro trains are driven by human drivers, but automated trains also exist, in, for example, London (the Victoria Line), Singapore, and Paris. This is not a recent invention; operation of trains on the Victoria Line has been automatic since its opening in 1968. However, in common with most systems, an operator is still carried in a cab at the front of the train. The VAL (véhicule automatique léger) of Lille, inaugurated in 1983, provided the first driverless underground system. Other driverless lines now include the line 14 (Meteor) of the Paris Metro, opened in 1998. The Docklands Light Railway (1987) in London, whilst for the most part not underground, is also driverless. See also People mover.
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 structural columns of steel, concrete, or, in the oldest systems, cast iron (most of the below-ground part of the New York Subway system is constructed in this manner, known as cut-and-cover). Another usual way is to dig the tunnels (often with tunnelling shield) beneath previously occupied subterranean space, through native bedrock, and seal the tunnels from leakage of ground water with concrete.
Underground systems use a variety of technologies. Most systems run on steel wheels and rails, although many modern systems use rubber tires and concrete rollways. (The Montreal metro was the first completely rubber-tired underground system.) Power may be supplied either by means of a third rail (New York) or by overhead lines (Madrid). Systems may be underground, at grade, elevated, or a mix as in the Paris metro. Some systems use light rail; other cities' systems are hybrids wherein a tramway moves underground in the city centre.
Underground systems need constant investment from the public authority, to avoid disasters like King's Cross fire in London's underground.
An exception to the rule that underground trains are for public transportation is the Post Office Railway, a driverless, underground railway in London that was used exclusively to transport mail between sorting offices, though it is now "mothballed".
The oldest subway tunnel in the world is the Atlantic Avenue Tunnel in Brooklyn, New York, built in 1844.
The London Underground was the first more extensive system.
Boston has the oldest subway system in the United States, the green line.
A person with a devoted interest in these systems is a metrophile.