History of the Berlin U-Bahn

During the nine years after the German Empire was founded, the city's population grew by over one-third and traffic problems increased.

Near the end of the Battle of Berlin in early May 1945, the north-south tunnel was demolished and broad sections of the subway flooded; reconstruction of the prewar system lasted until 1950.

The U-Bahn was built in three major phases: At the end of the 19th century, Berlin planners were looking for solutions to the city's increasing traffic problems.

Industrialist and inventor Werner von Siemens suggested the construction of elevated railways, and AEG proposed an underground system.

Berlin's administrators feared that an underground metro would damage the sewers, therefore favoured an elevated railway which would follow the path of the former city walls.

An extension along the Berliner Straße (today's Otto-Suhr-Allee) was proposed, but there was concern that having the Berlin-Charlottenburg tram alongside parallel traffic was not sensible.

For the route westward to Charlottenburg, the following stations were to be built: Emperor Wilhelm II opened the line on 16 March 1908, and the inaugural trip became known as the "minister ride".

Since the Prussian forest administration wanted to sell their portion of the Grunewald profitably, the overhead railway company purchased 14 hectares of land to build their new operating workshop and began an extension to the stadium station (Olympia-Stadion).

Operated as a tram from Warschauer Brücke to the Central-Viehhof metro station, it was acquired in 1909 by the city and opened on 1 January 1910.

Three and a half weeks later, on 27 July, the northern extension to Schönhauser Allee (originally called Nordring) and the S-Bahn was opened.

In the summer of 1907, Wilmersdorf's elevated railway company suggested an underground line to Nürnberger Platz and (if the new city would pay for it) Breitenbachplatz.

The 10-kilometre (6.2 mi) Kurfürstendamm and Dahlem lines, which opened on 12 October 1913, were the last U-Bahn sections built before World War I.

The 1913 route was: The early network ran mostly east to west, connecting Berlin's more-prosperous areas to increase profitability.

The new lines would use wider carriages on the same 1,435 mm (4 ft 8+1⁄2 in) standard gauge track, increasing passenger capacity, and became known as the Großprofil ("large profile") network.

Construction of the Nord-Süd-Bahn (North-South Railway), connecting Wedding in the north to Tempelhof and Neukölln in the south, had begun in December 1912 but halted for the First World War.

The Nollendorfplatz station was completely rebuilt to incorporate the Schöneberg line (present-day U4), which was independently operated after its 1920 takeover by the city of Berlin.

In 1926, however, Prussia wanted to hand over the route free of charge (and debt); the state offered the Sommerfeld Group land and construction costs for an extension to Krumme Lanke.

The spur line's stations are: The section opened on 22 December 1929, and the Krumme Lanke entrance is one of Alfred Grenander's late works.

Spandau, Gatow, Kladow, Lichterfelde, Marienfelde, Weißensee, Karlshorst and Lankwitz would make up a new line, portions of which later became the U7.

Although the U-Bahn was kept running as long as possible, operations shut down on many sections; the Grenzallee station and its adjoining tunnel were leased to an armaments company.

Another, smaller leak in the tunnel ceiling under the Spree was caused by German forces exploding the Ebertsbrücke Street bridge crossing at the river near the tunnel or, months later, by Soviet military engineers who created the hole while clearing the Spree of large concrete obstacles left by the bridge explosion.

The sections were quickly reopened beginning on 14 May 1945; most operated as shuttles, due to damaged trains and the lack of crossovers.

In response, the Soviets occupied the city's transit routes on 24 June in the Berlin Blockade; road, rail and ship connections to the surrounding area were not affected.

In 1953, loudspeakers on the trains warned of the approaching border; East Germans entering the western sector became subject to restrictions by their government.

During the June 1953 uprising, the following sections were closed: The day after the strike, service on lines A and C with connections to Nordbahnhof and Friedrichstraße.

To circumvent East Berlin and provide rapid transit to densely-populated areas in Steglitz, Wedding, and Reinickendorf, a third north–south line was needed.

GDR Interior Minister Karl Maron ordered the following changes to the U-Bahn: In West Berlin, housing developments in Britz and Rudow necessitated an extension to the CI line; the Grenzallee–Britz-Süd connection opened on 28 September 1963.

The leases at Nollendorfplatz and Bülowstraße stations, which had been repurposed as a flea market and a Turkish bazaar to store the last West Berlin trams, expired in March 1991.

Reconstruction of the bridge began in 1993 (including the replacement of pillars damaged by World War II air raids), and the Warschauer Straße station was rebuilt after it was partially demolished during the GDR era.

It was extended on 4 December 2020 to Alexanderplatz, where it connects with the U5 to Hönow (creating three new stations in the city center: Berliner Rathaus, Museumsinsel and Unter den Linden).