An underground way or gallery; especially, a passage under a street, in which water mains, gas mains, telegraph wires, etc., are conducted.
An underground railroad, usually having trains powered by electricity provided by an electric line running through the underground tunnel. It is usually confined to the center portion of cities; -- called also tube, and in Britain, underground. In certain other countries (as in France or Russia) it is called the metro.
an urban railway, running short distances either underground or above, carrying passengers only.