I think it was the other way around. well, almost. In the initial post-war years there was no Berlin Wall of course, and the city was just basically one city divided into four occupation sectors (American, British, French and Russian). Trains thus ran between these zones more or less as if political differences didn't exist, and commuters might work in one sector and live in another. The entire S-Bahn system was initially run by the Russians under a special arrangement, with this duty subsequently being handed over to East Germany.
When the Berlin wall went up in 1961 (I think), Berlin became a divided city over night, and this was also felt by the the transit system, with for example streetcar lines being split into two over night as the wall was literally built over the tracks in several locations. Some parts of the S-Bahn system were similarly closed down at this point, or what had previously been thru lines, suddenly split into two. As a strange quirk, one line of the West German system cut through a section of East Berlin in a tunnel. The stations on this section were shut down and no trains stopped there. Emergency exits were maintained so that the tunnels could be evacuated into East Berlin in case of an emergency, but were fitted with alarms so nobody tried to use them going the other way, to escape out of East Berlin. East German security agents regularly patrolled the disused stations to dissuade escape attempts. The stations thus fell into a kind of time warp, with 1950s decorations, posters and signs surviving until they were re-opened in the 1990s.
In later years West Berlin became increasingly unhappy with the East Germans continuing to make use of their right to run their S-Bahn, and with them refusing to negotiate a potential change of ownership. This led to the S-Bahn boycott in which passengers avoided the system, causing ridership to plummet and forcing the East Germans to cut back services and close lines as they incurred increasingly heavy losses. Finally an arrangement was agreed in which the East Germans handed over all operations in West Berlin to a West German entity. The East Germans used the opportunity to turn over with it the most decrepit and oldest of the trains they had, basically meaning that in the early years of West Berlin control, only a skeleton service could be operated until new trains could be acquired.
It was not until after the fall of the Berlin wall and re-unification that a major attempt was made to repair the worst of the cold war damage, and reinstate lines that had been closed. Even so, there are still lines that have not been re-opened to this day, despite repeated reiterations of the intention to do so.
Basically correct, but a few notes:
With the split created by the Wall on August 13, 1961, two West Berlin S-Bahn lines ran segments in East Berlin. The one in the previous photo of the boys in the open doorway ran Wannsee<>Steglitz<>Friedrichstrasse<>Frohnau via the Nord-Sud tunnel. Friedrichstrasse, the hub of the unified network, remained "open" for transfers with other west lines and was a border crossing. The one in the previous photo of the dawn train crossing the Spree ran Wannsee<>Zoo<>Friedrichstrasse elevated via the 1882 Stadtbahn. As the border controls were organized in a complex manner that requires a big diagram, that made it possible for west passengers to transfer between the two S-Bahn lines without "entering the capital of Democratic Germany." Furthermore, it was possible to transfer for the West Berlin U-Bahn subway that passed under East Berlin.
Friedrichstrasse was also a stop for interzone express trains like the
East-West Express with its through Paris<>Moscow sleeper and the seasonal
Hoek-Warsaw Express with an Amsterdam<>Moscow sleeper, pulled by a Reichsbahn Pacific in June 1969 when I first saw it. These trains served West Berlin at the Zoo Station.
The Friedrichstrasse station also was a connecting point for spies who used employee-only access points for a rendezvous and a left luggage facility for dead drops. The latter had a comic moment when the Reichsbahn changed the counter hours with little notice, leaving an agent unable to pick up a dropped package as scheduled.
The S-Bahn was off-limits to GI's, but the two U-Bahn lines that passed under East Berlin were omitted from our instructions, so I made several trips on those lines. The easternmost U-Bahn line went through the Alexanderplatz station with no stops. That station had a single-track underground connection to the one large-profile East Berlin U-Bahn line.
At the so-called ghost stations, one guard circulated on the platform, covered by additional personnel in the rail staff breakroom, modified with gun slits. Trains had to slow as they passed the platforms and on one occasion in my time there, a drunk who had missed his stop opened a door and jumped out into East Berlin. He was sent back.
It is not entirely correct to say that emergency access points remained. For various reasons, there were restrictions. (One reason that not all stations were reopened right away.) It was not a big hazard, because ridership was low. The big hazard for a westerner was that they were in a police state for a few minutes. If there was an accident, they would have ended up in an East hospital. People with political issues avoided the east-west trains.
One reason the ridership was low was the boycott. However, the boycott began because of the Wall, long before the crumbling service and labor troubles led to the city takeover. Extensions of the U-Bahn in several places duplicated the S-Bahn. And, as often happens on takeovers, the U-Bahn management was not very interested in their new responsibilities. I'm not sure, but I don't think there were any lines abandoned till the city takeover. Certainly, when I was stationed there every West line was running, complete with Saturday night Owl service for handfuls of passengers.
The streetcar lines were cut at the border before the Wall was built. Passengers walked across the border. West Berlin streetcars were discontinued entirely in 1967. It is complicated to explain now because former East lines have been extended into former West streets. There were struggles in the merged systems, as it was really a takeover by the more sophisticated western management, bus and U-Bahn oriented. When I made my first reunion trip in 2002, I arranged to meet with counterparts at the BVG (city transit) and it was clear that the streetcar people were low priority. That has changed since, but not before attempts were made to cut back streetcar lines.
Regarding restoration work, more correctly it can be said that work is catching up on WWII and Cold War and city-ownership damage. A huge effort also was undertaken to retroactively conform with the German equivalent of the ADA, which with age I appreciate more and more.
1971 - Streetcar passengers transferred here until August 13, 1961. Rails reflect early light. Due to agricultural and industrial pollution, the sky was lit before dawn.
