U-Bahn, the Berlin-underground

At the end of the nineteenth century, solutions were being sought to the problems of traffic in Berlin and the neighboring communes, which were later annexed by the city. After several solutions had been studied and rejected, the first stretch of metro between Warschauer Straße and Zoologischer Garten with a branch to Potsdamer Platz was built and commissioned in February 1902. It was still essentially an underground train. Shortly afterwards, the municipalities of Wilmersdorf, Schöneberg and Charlottenburg (which were not yet part of the city of Berlin) began to plan their own line, which first was to travel between Dahlem in the south and Spittelmarkt in the north, and later be extended to Olympiastadion. This primary grid with a small gauge corresponds to the current lines U1, U2, U3 and U4.

After the First World War and the absorption of neighboring communes in Greater Berlin in 1920, plans for a subway line between Wedding and Tempelhof with a possible branch to Neukölln - were developed. This so-called North-South line constitutes the present line U6 except the branch of Neukölln integrated nowadays to the U7. AEG also undertook the construction of its own line (the GN-Bahn line U8) between Gesundbrunnen and Leinestraße in the neighborhood of Neukölln via Alexanderplatz. But the completion of these new lines was delayed until the late 1920s because of the economic crisis and the period of hyperinflation that Germany experienced between 1918 and 1923. In the 1930s a new line , the future U5, was built between Alexanderplatz and Friedrichsfelde east of the city center.

Information from Wikipedia