the.com/underground railway

means A secret network of people, routes, and safe houses that helped enslaved people escape to freedom in the 19th-century United States.

from Not a real railway at all — the name is borrowed metaphor. In the 1800s, as steam railroads were spreading and capturing the public imagination, people began describing the clandestine escape network using railway slang: 'conductors' (guides like Harriet Tubman), 'stations' or 'depots' (safe houses), and 'passengers' or 'cargo' (the escapees). The 'underground' part signaled its secrecy — invisible to slaveholders and law enforcement, operating outside the sanctioned, aboveground systems of the time. The term caught on precisely because the metaphor was so vivid: an organized, purposeful system of movement toward freedom, just one with no literal rails or engines.