What is the significance of the title, The Underground Railroad?

In North American history, the underground railroad is the name given to a loosely affiliated network of homes and other safe places that aided in the movement of escaped slaves from the American South to the North, and at times into Canada. The historical railroad was not an actual railroad: it was just a chain of stopping points, or stations, along the way. The novel turns that metaphorical railroad into a literal one, positing the creation of a series of tunnels, tracks, and railway cars that actually provided physical transportation for escaping slaves. This is an entirely fictitious creation: in historical fact, there was no such literal railroad. Both the history and fictionalized railroad, however, have symbolic meaning in common: to the escaping slave, and to those that helped them, the railroad meant / means freedom.