Stellwerk Fichtengrund is a decommissioned railway signal box on the outskirts of Berlin in Oranienburg, Germany, formerly in the GDR. It was built in 1964 by the GDR railway Deutsche Reichsbahn to operate the connection to, what was at the time, a secret rail track connecting the Prussian Northern Railway with the Heidekrautbahn. The track, which was omitted from maps during the GDR period, was used to test a new Telescopic Axle to allow train coaches to easily change from European to Russian track gauge. The prototype worked but was never put into mass production.