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...
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."
221 feet is the greatest depth..... below are some figures to give you an idea of what depths (literally) they had to go to:
Maximum depth below mean sea level Northern Line, just south of Waterloo...