1.  subway(n = noun.artifact) metro, subway system, tube, underground - 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'"
is a kind of railroad, railroad line, railway, railway line, railway system

2.  subway(n = noun.artifact) underpass - an underground tunnel or passage enabling pedestrians to cross a road or railway;
is a kind of tunnel

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]

subway, n.
1 a a tunnel beneath a road etc. for pedestrians. b an underground passage for pipes, cables, etc.
2 esp. US an underground railway.

N  journey, travel, traveling, wayfaring, campaigning, journey, excursion, expedition, tour, trip, grand tour, circuit, peregrination, discursion, ramble, pilgrimage, hajj, trek, course, ambulation, march, walk, promenade, constitutional, stroll, saunter, tramp, jog trot, turn, stalk, perambulation, noctambulation, noctambulism, somnambulism, outing, ride, drive, airing, jaunt, equitation, horsemanship, riding, manege, ride and tie, basophobia, roving, vagrancy, pererration, marching and countermarching, nomadism, vagabondism, vagabondage, hoboism, gadding, flit, flitting, migration, emigration, immigration, demigration, intermigration, wanderlust, plan, itinerary, guide, handbook, guidebook, road book, Baedeker, Bradshaw, Murray, map, road map, transportation guide, subway map, procession, cavalcade, caravan, file, cortege, column, vehicle, automobile, train, bus, airplane, plane, autobus, omnibus, subway, motorbike, dirt bike, off-road vehicle, van, minivan, motor scooter, trolley, locomotive, legs, feet, pegs, pins, trotters, traveler, depot, railway station, station, traveling, ambulatory, itinerant, peripatetic, roving, rambling, gadding, discursive, vagrant, migratory, monadic, circumforanean, circumforaneous, noctivagrant, mundivagrant, locomotive, wayfaring, wayworn, travel-stained, on foot, on horseback, on Shanks's mare, by the Marrowbone stage: in transitu, en route, Int, come along!.

subway fare, subway station, subway system, subway token, subway train