Dictionary
un'derground rail'road
1. Also called
un'derground rail'way. a railroad running through a continuous tunnel, as under city streets; subway.
2. (often caps.) U.S. Hist.(before the abolition of slavery) a system for helping fugitive slaves to escape into Canada or other places of safety.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.