Torn Curtain was filmed mostly at Universal Studios with second unit filming in Germany. sixcyl's screen captures show two different buses.
The first capture shows a process shot with back projected footage from Germany. The bus has a real destination blind with reflections on the glass, and there is a shadow cast on the blind behind the window.
The bus in the second capture has the same registration number but the actual plate is different. The destination blind is just pasted onto the window.
I imagine that in real life people going from Leipzig to Berlin go by train, not by bus.