I think his point was, and I think he is right, that the DDR authorities
allowed anyone to cross the wall who was either not one of their own
nationals or a national of another country (ie the Eastern Block) with whom
they had an agreement to stop travel.

The West German authorities did not check entries to West Berlin from East
Berlin, presumably because of the four powers agreement which said that West
Berlin was made up of British, American and French Sectors.

Nationals of anywhere outside the Eastern Block could arrive at Schonefeld
Airport and get a transit visa to Friedrichstrasse and cross into the West
there. If I recall it resulted in quite a few Palestinians living in West
Berlin at the time.


As an aside there's quite a good article on Friedrichstrasse Station (in
English) on Wikipedia at


http://en.wikipedia.org/wiki/Berlin_...C3%9Fe_station

which has diagrams and other periond information,