A rapid transit rail transport system, or a train in such systems, generally underground and serving a metropolitan area.
An urban rapid transit rail transport system, normally with lighter track and light trains (i.e. an S-Bahn or similar), or a train in such systems.
A metropolitan area.
metropolitan
Synonym of subway: a railway that is under the ground.
A movement or organisation of people who resist artistic convention.
A movement or organisation of people who resist political convention.
Regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines).
Below the ground.
Secretly.
Hidden, furtive, secretive.
Below the ground; below the surface of the Earth.
Outside the mainstream, especially unofficial and hidden from the authorities.
To route electricity distribution cables underground.