I visited West Berlin in 1986. I vividly remember traveling one of the U-Bahn lines that ran north from Friedrichstrasse station (which was in the West). The stations that were in the East were closed to passengers, and the trains did not stop there, but there was always a soldier or two on the platforms, and the stations were poorly lit, decrepit, and just plain scary. The U-Bahn lines that were completely in the East remained open, but disconnected from the West. These stations are called "Geisterbahnhof", or "ghost train stations"

In other words, the only way for tourists like me (especially Americans) to visit the East was to pass through Checkpoint Charlie at Friedrichstrasse.

Try this link: en.wikipedia.org/wiki/Ghost_station