1977 - West Berlin map of entire U-Bahn system - The BerlinWall (Sektorengrenze/Sector Boundary) is a gray "slashed" line. To its right, stations on a thin black line can only be reached using the the East BGV. Stations on lines 6&8 in the East are crossed out. West trains trains do not stop at these "Geisterbahnhofe/ghost stations," patrolled by DDR border guards to prevent unauthorized crossings to the west. Friedrichstraße station was open - an official checkpoint between the two sectors.