Underground and Underground railway

Underground noun - A secret organization in a conquered country fighting against enemy forces.

Underground railway and underground are semantically related in subway topic. In some cases you can use "Underground railway" instead a noun "Underground", 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.

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