the Underground Railroad
/ði ˌʌndəɡraʊnd ˈreɪlrəʊd/
/ði ˌʌndərɡraʊnd ˈreɪlrəʊd/
- a secret system used in the US before the Civil War for helping thousands of slaves to escape to the free northern states or Canada. The slaves were called 'passengers', the people who helped them were 'conductors', and the slaves hid in 'stations' (safe houses) along the way.