Trolley line and Underground railway

Trolley line noun - A transit line using streetcars or trolley buses.

Underground railway and trolley line are semantically related in train line topic. In some cases you can use "Underground railway" instead a noun phrase "Trolley line".

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

Trolley line and underground railway are semantically related in train line topic. You can use "Trolley line" instead a noun phrase "Underground railway".