The S-Bahn North–South line encompasses the route from Bornholmer Straße and Gesundbrunnen via Friedrichstraße and Anhalter Bahnhof to Papestraße (today Südkreuz) and Schöneberg.

The construction of an elevated line like the Stadtbahn was ruled out because of extensive building along the route, and a tunnel would have been highly problematic in the steam age.

Because of the number of underground lines to be crossed, the need to pass under rivers and streams meant that tight curves, small-profile tunnels and steeper inclines than normal were incorporated into the planning parameters.

[1][2] This stretch was extended to the new underground Potsdamer Platz station in April 1939, and the connection to the Southern suburban lines via Anhalter Bahnhof was opened on 9 October 1939.

Those Ringbahn trains would reverse in a staging area north of the underground Potsdamer Platz station, which was designed as the upper level of the two-floor tunnel underneath the north–south street formerly and later called Ebertstraße but back then Hermann-Göring-Straße.

This two-level design was chosen out of the scarcity of space, and in order to allow a later extension to the then Lehrter Bahnhof and its suburban lines to Nauen and Wustermark.

This stabling and reversal yard with its inherent possibility of extension north could be used as part of the S21 line to the new Berlin Hauptbahnhof, then on to the northern section of the Ringbahn.

However, Oranienburger Straße, Nordbahnhof and Unter den Linden stations began the modernisation programme to remove armed guards and out-of-bounds signage, and to do thorough cleanup work, with Oranienburger Straße reopening on 2 July 1990, while Nordbahnhof, together with Unter den Linden reopened on 1 September 1990.

Bornholmer Straße, a station that straddled the former "sector boundary" (between East and West Berlin), reopened on 22 December 1990, and there were no checkpoint and armed guards before then.

The building of the new heavy-rail tunnel to the Hauptbahnhof and the connected electromagnetic effects of the overhead wiring meant that a new compatible signalling system had to be installed.

The North–South Line was the last section of the Berlin S-Bahn network to have the automatic Block signalling of type AB 37 and Sv-Signals with aperture relay installed.

The restrictions in carriage height and length continue to limit the trains that can be used, with only rolling stock specially developed for the Berlin S-Bahn (as of February 2025 consist of classes 270/485, 480, 481/482 and 483/484) allowed to use the tunnel.

Except in a few details, e.g. the mosaic of the emblem in the northern distribution area of S-Bahnhof Potsdamer Platz, with its exit to the New Reich Chancellery, the stations do not follow typical National Socialist Architecture.

During the renovation of Anhalter Bahnhof, the BVG (on behalf of the Senate Construction Administration) attempted to restore the original look of the station and placed enamelled tin plates on the walls.

Since 8 August 2009, in connection with the opening of the interchange with the new U55 line, Unter den Linden has been renamed Brandenburger Tor, although the original name tiles remain on the station walls.

The building work for the northerly section up to the Hauptbahnhof, which will be funded by the German Federal Government via the Community Transport Financing [de] law, costed 24 million Euro.

[6] Some preparatory work has already been completed, including the diversions at the Ringbahn stations of Westhafen and Wedding, the preservation of the route during the building of the Hauptbahnhof and the Tiergartentunnel, along which the new line will run.

To the south of the Hauptbahnhof the line will continue easterly of the Reichstag building and then crossing the U55 to the west of Brandenburger Tor where it will join the old North–South Tunnel.