Although a few parts of the network involved tunnels or underground hiding places, most of the "underground railroad" consisted of houses and farms of anti-slavery Southerners. They used these...
Some actually were. The term "underground" refers to being hidden, unseen or concealed. Partisans usually functioned in small groups, so if captured they wouldn't be able to give away information on...
No, it was a secret network to the North for escaping slaves in the South prior to the US Civil War. It was a step-by-step passage created by escaped slaves and abolitionists, including sympathizers...
The term "underground" in underground railroad means secret. It also was not a road. It was more like a trail. So I guess if you were to say it literally, you would say "secret trail."