underground railroad

WordReference Random House Unabridged Dictionary of American English © 2026

un′derground rail′road, 

1. Rail Transport, Transport Also called un′derground rail′way. a railroad running through a continuous tunnel, as under city streets;
   subway.
2. American History (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.
• 1825–35

Collins Concise English Dictionary © HarperCollins Publishers::

underground railroad noun

1. (often capitals) (in the pre-Civil War US) the system established by abolitionists to aid escaping slaves