Subway system and Underground railway

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

Underground railway and subway system are semantically related in subway topic. In some cases you can use "Underground railway" instead a noun phrase "Subway system", when it comes to topics like metro.

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

Subway system and underground railway are semantically related in subway topic. You can use "Subway system" instead a noun phrase "Underground railway", if it concerns topics such as metro.