Construction of the North-South Underground: 1901-1930

Map of the former Nord-Süd Bahn in 1930

Around 1901, the city of Berlin planned to build an underground railway line below Friedrichstraße to connect the north to the south. Werner von Siemens also had plans for a north–south line, under Nobelstraße, at the same time, but permission for these was declined on the grounds that public transport should be in municipal ownership. Consequently, Berlin started the construction of the Nord–Süd-Bahn (North–South railway) to link Wedding and Tempelhof (as of 2007, part of the U6), with a branch to Neukölln.

World War I made the construction work difficult, and finally stopped it completely. In 1919, work started again, but not without further complications. In 1921, during the time of hyperinflation, filling up the existing tunnels was considered as financial turmoil hit hard. The construction work was continued, however, and the first tunnel section from Hallesches Tor to Stettiner Bahnhof (later renamed Naturkundemuseum) was opened on 30 January 1923.

The history of the U7 began with the construction of the branch to Neukölln, when the stretch from Hallesches Tor to Gneisenaustraße was built; it opened on 9 April 1924. With inflation still taking its toll, the construction work proceeded in small steps only. The extension to Hasenheide station, named after a nearby park (and later renamed Südstern), followed on 14 December of the same year. As the financial situation of Germany and Berlin improved, so too did the underground railway construction, including the branch to Neukölln.

At Hermannplatz station, which resembles something of an U-Bahn cathedral, the first underground rail–rail crossing in Berlin was developed; transfer to the GN-Bahn (later the U8) was intended at this station. The station is also the first Berlin underground station to use escalators. The section from Hasenheide to Bergstraße (later Karl-Marx-Straße) was put into operation on 11 April 1926. The final stage of the Neukölln branch at that time, the 1.5 kilometre-long (0.9 mi) extension to Grenzallee, was put into operation on 21 December 1930—the same day of the opening of what would become the U5 line—during one of the largest underground opening celebrations. Underground passengers could then travel from Seestraße (U6), through the city centre, to either Tempelhof or Grenzallee.

The post-war underground lines were marked out from their 1901–1914 predecessors by their larger tunnels and trains, in order to provide greater competition with the privately run Berliner Hochbahngesellschaft (English: Berlin Elevated Railway Company). The new trains and tunnels, which were wider but used the same standard-gauge track, were described as Großprofil (English: large profile).

Connection to Rudow

Rudow terminus station platform.

Following World War II—in which many of Berlin's residences were destroyed—large new housing developments were needed. Britz and Britz-Buckow-Rudow (BBR; later renamed Gropiusstadt), which were established in the south of West Berlin, required a new rapid-transit railway connection to the West Berlin city centre; the construction work for the underground railway began on 2 November 1959.

The track from Grenzallee to Britz-Süd opened on 28 September 1963.

Construction began from Britz-Süd to Rudow on 2 January 1965. Travel to Zwickauer Damm was made possible on 2 January 1970, and Rudow received connection to Line 7 on 1 July 1972.

Extension to the west

The branch station Belle-Alliance-Straße (later renamed Mehringdamm), which opened in 1924, had three tracks: a connection to Tempelhof led from the first platform; to Neukölln from the second; and to the city centre from the third. A separation of the Neuköllner branch from the North-South Line was decided upon in order to avoid an overload on the transfer station Hallesches Tor. Therefore, Line 7 was extended west from Belle-Alliance-Straße. The construction work began in the middle of 1962, and was completed on 26 February 1966 with the link to Möckernbrücke station. Beside the construction of Möckernbrücke station, changes to Mehringdamm station—which would become a transfer point between unbranched lines—were necessary.

The further extension of Line 7 was accomplished with subsidy funds. The construction of the track from Möckernbrücke to Fehrbelliner Platz started on 1 July 1962, and finished on 29 January 1971.

Altstadt Spandau: Last station before the terminus Rathaus Spandau

In three stages to Spandau: 1960-1984

At the end of the 1960s, ideas about an underground railway line to Spandau arose again. The planned extension to Line 7 through Mierendorffplatz, Jungfernheide, and Nonnendammallee was chosen to improve public transportation to Siemensstadt—important for its many jobs. Already at the time, there were plans to build a subway to Spandau. First steps were taken with the extension of the line of today's line U2 to Ruhleben. The terminus Ruhleben is located directly on the district border to Spandau. Feeder and distribution traffic, until 1967 by tram, replaced by several bus lines, provided solely for the development of Spandau on the subway station Ruhleben.

The construction work for the section Fehrbelliner Platz–Richard-Wagner-Platz began in 1969 which is H-West-II. By 1970, the then shortest and less frequented subway line of Berlin, line 5, oscillated between Deutsche Oper and Richard Wagner Platz. Their old route was shut down for passenger traffic, and a new station was built several meters below the old one. The remaining tunnels could be converted into an operating section, making it the second exchange line between small and large profile after the Waisentunnel at Alexanderplatz station. The route continues under the Brandenburgische Straße and crosses the Kurfürstendamm at Adenauerplatz, where a train station was also built. The station was created as a crossing station as it was (and officially still is) planned to extend the current U-Bahn line U1 from Uhlandstraße to Adenauerplatz. After Adenauerplatz, the track swings under Wilmersdorfer Straße and passes under the Stadtbahn.

Wilmersdorfer Straße was constructed with a pedestrian passage to the Charlottenburg S-Bahn station. At Bismarckstraße, a new underground railway station had to be built, since the pre-existing tunnel of Line 1 (later renamed U2) was in a bad condition and necessitated extensive repair works. After Bismarckstraße, 23 houses had to be underpinned, therefore the shield drive was used for tunnelling this section. The stage from Fehrbelliner Platz to Richard-Wagner-Platz was put into operation on 28 April 1978.

Earthworks and some motorway construction began at Charlottenburg-Nord on 3 July 1967, which is located at Siemensstadt. Construction began for the section to Rohrdamm from 1973. The construction work for the next section towards Spandau began in 1973. The route leads north of the station Richard-Wagner-Platz further under the Sömmeringstraße. Halfway to Mierendorffplatz, the Spree is underpassed using the caisson construction method. After a few meters, the station follows Mierendorffplatz. North of the station, the route makes a wide berth to reach the existing S-Bahn station Jungfernheide. Here, similar to the station Schloßstraße, the platforms were built on two levels. The other tracks are kept until today for a possible extension of the U5, which is to lead from Lehrter to Tegel Airport via Turmstraße and Jungfernheide. However, the plan was cancelled due to the budget situation, the longer length of the extension north of Jungfernheide station due to the planned closure of Tegel Airport. Trams will be replacing the U-Bahn extension. The finished extension is now used for firefighting exercises and is no longer usable for any future line. The extension to Rohrdamm followed on 1 October 1980, providing the district of Spandau with its first underground connection.

The costs of the extension to the Spandau city centre rose to unexpected heights: it consumed 680 million German marks as a result of the need to undercut the Havel river and the swampy ground in the surrounding area. Construction began on 4 July 1977. Several variants were investigated. One plan was to lead the subway through the Haselhorster village center, with a downside that the Havel had to be undercut at one of its widest points. Another variant was that the subway would cross the existing Spandau suburban railway of the S-Bahn and the subway would be led from the south to Spandau city hall. This plan crystallized very late, as the 200-kilometre (120 mi) plan envisaged to lead the U7 over Falkenseer Platz into Falkenhagener Feld. This resulted in the disadvantage that the Spandau old town could not be connected, which was the main goal of the extension. Finally, they decided to continue the route along the Nonnendammallee or the street Am Juliusturm, south past the Spandau Citadel and then directly to the old town and on to the Spandau town hall, with the development of Falkenhagener Feld taken over by the extension of the small profile route of the line U2 of Ruhleben. On 1 October 1984, the last section of the U7 from Rohrdamm to Rathaus Spandau was opened, with the then Federal Chancellor of Germany, Helmut Kohl, in attendance.