The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

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]