Underground Railroad (n.)youdaoicibaDictYouDict[Underground Railroad 词源字典]
"network of U.S. anti-slavery activists helping runaways elude capture," attested from 1847, but said to date from 1831 and to have been coined in jest by bewildered trackers after their slaves vanished without a trace. Originally mostly the term for escape networks in the (then) western states of the U.S.[Underground Railroad etymology, Underground Railroad origin, 英语词源]