subway

n. 1. An underground way or gallery; especially, a passage under a street, in which water mains, gas mains, telegraph wires, etc., are conducted. [ 1913 Webster ]

2. 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. [ PJC ]