Opened in 1902, the U-Bahn serves 175 stations[1] spread across nine lines, with a total track length of 155.64 kilometres (96 miles 57 chains),[3] about 80% of which is underground.

Although the system remained open to residents of both sides at first, the construction of the Berlin Wall and the subsequent restrictions imposed by East Germany limited travel across the border.

The elevated railway company did not believe such a line would be profitable, so the city built the first locally financed underground in Germany (intentionally using standard of Kleinprofil Berlin U-Bahn rolling stock).

The city also mandated that new lines would use wider carriages—running on the same, standard-gauge track—to provide greater passenger capacity; these became known as the Großprofil ("large profile") network.

[citation needed] Construction of the Nord-Süd-Bahn ("North-South railway") connecting Wedding in the north to Tempelhof and Neukölln in the south had started in December 1912, but halted for the First World War.

[citation needed] In 1912, plans were approved for AEG to build its own north–south underground line, named the GN-Bahn after its termini, Gesundbrunnen and Neukölln, via Alexanderplatz.

[citation needed] The war had damaged or destroyed much of the network; however, 69.5 km (43.2 mi) of track and 93 stations were in use by the end of 1945, and the reconstruction was completed in 1950.

In order to circumvent East Berlin, and provide rapid-transport connections to the densely populated areas in Steglitz, Wedding, and Reinickendorf, a third north–south line was needed.

The U2 was split into two sections, and for the north–south lines, trains were not allowed to stop for passengers and become Geisterbahnhöfe ("ghost stations"),[7] patrolled by armed East-German border guards.

The result was a station with a restrained blue-grey tiled colour-scheme and Berlin's first underground shopping facilities, designed by Alfred Grenander.

The redesign also featured a new entrance building, which blended into the grand architectural styles of Wittenbergplatz and the nearby KaDeWe department store.

The Berlin U-Bahn mostly runs on an honor system and has been noted for its relative lack of turnstiles in its stations; instead transportation agents will inspect tickets and fine fare evaders.

Provided either by the Job Center (Arbeitsamt) for out-of-work residents or by the Sozialamt for people who cannot work or are disabled, the S-Class ride-passes normally restrict travel to the AB zones and must be renewed (a new pass purchased at a non-automated location) on the 1st of each month.

On the tram, S-Bahn and U-Bahn, a proof-of-payment system is used: there are random spot checks inside by plain-clothed fare inspectors who have the right to demand to see each passenger's ticket.

Another concrete wall separates this tunnel, which now houses a transformer for an electricity supplier, from the never-completed Oranienplatz Station which is located partially under the square of the same name.

Although a Berlin map dated 1946 shows the station renamed as Bersarinstraße after the Soviet General responsible for restoring civil administration of the city, this name was used later at another location.

The connection from Innsbrucker Platz station to the depot was severed when a deep level motorway underpass was constructed in the early 1970s; however, the continuation of the tunnel at Eisackstraße is still in existence for a distance of 270 metres and now ends at the former junction to the workshop of the Schöneberg line.

After the construction boom that followed the reunification of the city, enthusiasm for further growth has cooled off; many people feel that Berlin's needs are adequately met by the present U- and S-Bahn.

There are several long-term plans for the U-Bahn that have no estimated time of completion, most of which involve closing short gaps between stations, enabling them to connect to other lines.

As of 2017, Class IK Kleinprofil trains are in operation on the Großprofil line U5, after refurbishing the existing F79 rolling stock was deemed unfeasible.

Designated B-I, the cars were 13.15 m (43 ft 1+11⁄16 in) long and each had three sliding doors; the large elliptical windows at the front of the train earned them the nickname, Tunneleulen (tunnel owls).

The first D-type trains, manufactured in 1957, were built from steel, making them very heavy and less efficient; however, the DL type that followed from 1965 used metals that were less dense, allowing a 26% reduction in weight.

[citation needed] The most severe accident occurred at the original Gleisdreieck (rail triangle), where the main and branch lines were connected by switches that allowed the tracks to cross.

On 8 July 2000, the last car of a GI/I train suffered a short circuit, burning out at the rear of the Deutsche Oper station.

[citation needed] The Portuguese Ambassador, João Diogo Nunes Barata, presented the BVG with azulejos (tiled paintings), specially designed for the station, by the artist José de Guimarães.

[14] Whether set in Berlin or elsewhere, the U-Bahn has had at least a minor role in a large number of movies and television programmes, including Emil and the Detectives (2001), Otto – Der Film (1985), Bang!

Alexanderplatz station plays an essential role in Berlin Alexanderplatz—a film of thirteen hour-long chapters and one epilogue—produced in 1980 by Rainer Werner Fassbinder, based on the book by Döblin.

Sandy Mölling, former singer of the pop band No Angels, shot the video for her single "Unnatural Blonde" in the U-Bahn station Deutsche Oper.

Kate Ryan, Overground, Böhse Onkelz, Xavier Naidoo, Die Fantastischen Vier, and the DJ duo Blank & Jones have all used the U-Bahn and its stations for their videos as well.

They used the names of real stations that, in the context of underwear, appeared to be mild sexual double entendres: men's underpants bore labels with Rohrdamm (pipe dam), Onkel Toms Hütte (Uncle Tom's Cabin), and Krumme Lanke (crooked lake); the women's had Gleisdreieck (triangle track), and Jungfernheide (virgin heath).