WordNet (r) 3.0 (2006):

underpass
    n 1: an underground tunnel or passage enabling pedestrians to
         cross a road or railway [syn: underpass, subway]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "underpass":
   access, aisle, alley, ambulatory, aperture, arcade, artery, avenue,
   channel, cloister, colonnade, communication, conduit, connection,
   corridor, covered way, defile, exit, ferry, ford, gallery, inlet,
   interchange, intersection, junction, lane, opening, outlet,
   overpass, pass, passage, passageway, portico, railroad tunnel,
   traject, trajet, tunnel