Lines S3, S5, S7, S9, and S75 are east–west lines using the Stadtbahn cross-city railway. The western termini are located at Potsdam and Spandau, although the S5 only runs as far as Westkreuz and the S75 to Warschauer Straße. The eastern termini are Erkner, Strausberg Nord, Ahrensfelde, and Wartenberg. The S9 uses a connector curve (Südkurve) at Ostkreuz to switch from the Stadtbahn to the south-eastern leg of the Ringbahn. Another curve, the Nordkurve to the North-eastern Ringbahn, was originally served by the S86 line, but it was demolished in preparation of the rebuilding of Ostkreuz station and was not rebuilt afterwards. Both connector curves were heavily used in the time of the Berlin Wall, as trains coming from the North-Eastern routes couldn't use the West Berlin North–south route and the Southern leg of the pre- and post-Wall Ringbahn was in West Berlin.
