Subway and Underground railroad

Subway noun - An electric railway operating below the surface of the ground (usually in a city).

Underground railroad and subway are semantically related In some cases you can use "Underground railroad" instead a noun "Subway". popular alternative

Underground railroad noun - Secret aid to escaping slaves that was provided by abolitionists in the years before the American Civil War.

Subway and underground railroad are semantically related in secret cooperative network topic. You can use "Subway" instead a noun phrase "Underground railroad".