Friedrichstraße 

This had been a border checkpoint between west and east until 1990. It has been the only station where the S-Bahn (=commuter train) from West Berlin stopped while passing under a part of the eastern section. It looked completely different then - there had been an inscription "Berlin - Hauptstadt der DDR", a big eastern geman coat of arms and a shop where one could buy communist literature.

See: http://en.wikipedia.org/wiki/Berlin_Friedrichstra%C3%9Fe_railway_station#Border_crossing_during_the_cold_war

and

http://en.wikipedia.org/wiki/Ghost_station#Particular_stations

I entered East Berlin through this station two times, 1988 and June 1989. Some months after my second visit, the GDR Regime broke down - I'd never imagined that at this time...!