If not, how were they effectively prevented when they were able to be built even in some prison camps?
There were and still are: the U-Bahn. Several stations were bricked up and their tunnels narrowed so that at least one line from the west could still run under Alexanderplatz without being rebuilt.
In 1987 or so there was a TV series about subway systems in various cities -- New York, Seoul, Madrid, Toronto, London, et cetera. I had a heavy fascination with subways, so I taped them and have lo-rez VHS of them somewhere. The one about Berlin is a fascinating time capsule right before the Wall fell. The closed station turned into a beach scene sticks in my mind.