From WordNet (r) 3.0 (2006) [wn]:
subway
n 1: an electric railway operating below the surface of the
ground (usually in a city); "in Paris the subway system is
called the `metro' and in London it is called the `tube' or
the `underground'" [syn: metro, tube, underground,
subway system, subway]
2: an underground tunnel or passage enabling pedestrians to
cross a road or railway [syn: underpass, subway]
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]