|(often capitals) (in the pre-Civil War US) the system established by abolitionists to aid escaping slaves|
A network of houses and other places that abolitionists used to help slaves escape to freedom in the northern states or in Canada before the Civil War. The escaped slaves traveled from one “station” of the railroad to the next under cover of night. Harriet Tubman was the most prominent “conductor” on the Underground Railroad.
A secret network for moving and housing fugitives, as in There's definitely an underground railroad helping women escape abusive husbands. This term, dating from the first half of the 1800s, alludes to the network that secretly transported runaway slaves through the northern states to Canada. It was revived more than a century later for similar escape routes.