Just to add a bit more info...I believe that in the beginning, after the war, when the western Allied powers set up the flight arrangement, each (England, US & France had flights from their occupation zone to Berlin(tegel), as well as from Paris, London and Frankfurt(US)). I am not sure when it evolved to the state that existed in the late 80's thru early 1990 or 91.
When we visited Germany (& Berlin) in late winter 1990, we flew the Pan AM IGS from Frankfurt to TXL. I will always remember that , as it was my last PA flight before they went under. Also will never forget the differnce between E and W Berlin at Night when taking off from TXL.