Complex sentence: These were allowed to pass through East Berlin without stopping at any of the stations, which were closed with the exception of Friedrichstraße, used as a transfer point (between U6 and the West Berlin S-Bahn system) and a border crossing into East Berlin.
Split into simpler sentences: These were allowed to pass through East Berlin without stopping at any of the stations, which were closed. Friedrichstraße was the exception because it was used as a transfer point between U6 and the West Berlin S-Bahn system, and a border crossing into East Berlin.