Were is underground?

of corse NOT.Well, technically, yes.

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...

It meant it was a secret network of safe-houses for smuggling slaves into Canada.

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."

Yes, there still is slavery and an underground so to speak. Usually it is linked to international prostitution