At the risk of continental thread drift, I'll try to answer that reasonable question in brief.
In 1969 our Soviet colleagues at Marienborn (Checkpoint Alpha) were being levied for assignments in the border war with China. (We were being levied for the Vietnam War, our British regiment was being shipped to Northern Ireland, and our French comrades were off to Africa for their next civil war.) Information from all kinds of sources higher up than us led to the conclusion by the western allies that the Soviet leadership wanted to get the constant Berlin confrontations off the table. In turn the Soviets had figured out that we were talking with China, the people who were sniping at them. This situation led to the Four-Power Treaty on the Status of Berlin and there were no more Berlin crises till 1989.
The three sleeper lines, Frankfurt-Berlin, Bremerhaven-Berlin, and Bremerhaven-Frankfurt were the last scheduled operations of what had been a network of Army Transportation Corps trains reaching from Paris to Salzburg and Berlin in 1945.
The two trains to Berlin ran on the original Royal Prussian Railway through Magdeburg and Potsdam on what became the Main Line of the Cold War through the Soviet Zone of Germany. There also was a fortnightly U.S. freight train. The same single-track line carried scheduled British and French military trains, Soviet trains, interzone express trains, a solid mail train for the West Berlin post office, and local suburban trains.
There's more about the duty trains and the Four-Power negotiations in my website at:
https://www.berlin1969.com
The least inaccurate movie about this is Stop Train 349 (1963).
The most Hollywoodie is Berlin Express (1948).
Back to the thread, both movies portray Russians in the attitude of the times.
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