Looking into the North-South-Tunnel of the Berlin S-Bahn from the southbound platform of the Potsdamer Platz S-Bahn station. This was planned as the railway yard for S-Bahn trains using the Southern Ring hairpin bend, which turned at the above ground Postdamer Bahnhof (actually the separate "Potsdamer Ring- und Vorortbahnhof", on the Eastern edge of the main line station). What is seen here becomes a two-storeyed tunnel, with the actual tunnel leading north to the Brandenburg Gate station, in the lower level, coming from the outer tracks, and this staging area in the upper level. This upper level ends at Behrensstraße, and is designed in a way to be continued towards the North-West, the current Berlin Central Railway Station (Hauptbahnhof), or back then, the Lehrter Bahnhof