underground railroad(n = noun.act) underground railway - secret aid to escaping slaves that was provided by abolitionists in the years before the American Civil War;
is a kind of escape, flight

elevated railroad, railroad, railroad bed, railroad car, railroad engineer, railroad flat, railroad line, railroad man, railroad siding, railroad station, railroad terminal, railroad ticket, railroad tie, railroad track, railroad train, railroad tunnel, railroad vine, railroad worm, underground, underground insurance, underground press, underground railway