subway

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An underground urban railroad, usually operated by electricity.
2. n. A passage for such a railroad.
3. n. An underground tunnel or passage, as for a water main or for pedestrians.

Century Dictionary and Cyclopedia

1. n. An underground way; an accessible underground passage containing gas- and water-mains, telegraph-wires, etc.

Wiktionary

1. n. North America An underground railway, especially for mass transit of people in urban areas.
2. n. US A rapid transit system, regardless of the elevation of its right of way.
3. n. UK underground walkway, tunnel for pedestrians (called pedestrian underpass in US).
4. v. intransitive, US, informal To travel by underground railway.

GNU Webster's 1913

1. n. An underground way or gallery; especially, a passage under a street, in which water mains, gas mains, telegraph wires, etc., are conducted.
2. n. 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.

WordNet 3.0

1. n. an underground tunnel or passage enabling pedestrians to cross a road or railway
2. n. an electric railway operating below the surface of the ground (usually in a city)