Source : Webster's Revised Unabridged Dictionary (1913)

Subway \Sub"way`\, n.
   An underground way or gallery; especially, a passage under a
   street, in which water mains, gas mains, telegraph wires,
   etc., are conducted.

Source : WordNet®

subway
     n 1: electric underground railway [syn: {metro}, {tube}, {underground}]
     2: an underground tunnel or passage enabling pedestrians to
        cross a road or railway [syn: {underpass}]