The sign on the wall says "Abstellbetrieb. Halt Isolierste". I'm not entirely sure how this translates into English. My best guess is "Shunting movements: Stop, Neutral Section" or "Stop, Rail Gap", similar to the "flashing triangle" signal on London Underground.
The Berliner U-Bahn Museum is based in the former Olympia-Stadion signalbox. The museum is maintained by the Arbeitsgemeinschaft Berlin U-Bahn, and is normally open to the public on the second Saturday of each month.
berliner u-bahn museum
arbeitsgemeinschaft berliner u-bahn