From WordNet (r) 2.0 (August 2003) [wn]:
metro
n : electric underground railway [syn: subway, tube,
underground]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Subway \Sub"way`\, 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]