☆ in the U.S. before the Civil War, a system set up by certain opponents of slavery to help fugitive slaves escape to free states and Canada
See underground railroad in American Heritage Dictionary 4
noun
A secret cooperative network that aided fugitive slaves in reaching sanctuary in the free states or in Canada in the years before the abolition of slavery in the United States.
underground railroad A secret cooperative network engaged in the clandestine movement and housing of fugitives, such as children removed illegally from the custody of a parent charged with child abuse.