I think the long distance platform at Friedrichstraße was also
already open to West Berlin, but I am not sure of that.


The long distance platforms at Friedrichstrasse were available only to West
Berlin residents or anyone exiting East Berlin (with permits) through the
passport control. They also had to be travelling to the West (eg you could
not get on the Ost West express to Warsaw or beyond at Friedrichstrasse.
East Berliners had to board it at Ostbahnhof (then Hauptbahnof) and West
Berliners at Zoo or Wansee) because trains between Friedrichstrasse and
Ostbahnhof could only be used by passengers transiting the DDR.

The couple of daily trains that started at Zoo and went to Scandinavia via
the train ferries were not advertised to stop at Friedrichstrasse (for the
above reason), though they did for DDR customs checks.

Through tickets to Berlin were always issued to Berlin Stadtbahn and were
available to any destination along the Stadtbhan plus those stations
(Lichtenberg Karlhorst etc ) that were used by terminating trains within the
DDR and any station within or on the bits of the Ringbahn that were
operating.

Tickets via Berlin including the use of the S bahn between stations
(throughout both East and West Berlin but if you got off a train at
Friedrichstasse that terminated there you had to get pff there and get a
DDR transit visa at the customs post and then use the S Bahn to Ostbahnhof
(or wherever to continue your journey). On the through trains to the East
transit transit visas to Poland and beyone were issued on the trains (as an
alternative to the transit visa which they issued for transit from the West
to West Berlin)