underground railway (UK)—the underground railway transport system in a large city:
- The growing population made traveling in London very difficult, and it was thought that an underground railway would solve many congestion problems.
underground railway (U.S.)—(hist.) the chain of safe houses used by slaves escaping from the South prior to the abolition of slavery:
- Windsor was an important terminal of the Underground Railway. Fugitive slaves landing near this spot found in Canada protection under the British flag.