Before November 1989? I'm astonished.

Definitely. It was about August 1989. No sign of wall coming down at all.
They had DDR 40 in lights on a property in Alexanderplatz. Hard to believe
now that it never got much further!
Went up the funkturm as well- quite a good meal in the telecafe for £5. At
that time of course the West Berlin S Bahn was being operated by the BVB
(BVG) in West Berlin, at least those parts of it which were open.

The S Bahn trains on the Stadtbahn changed crew at Lehrter Stadtbahnhof, a
DR driver noticeably taking over from the BVB one for the short run into
Friedrichstrasse,

I don't think any such change occurred on the North Sud bahn at the time,