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Rapid transport system of Berlin, Germany
Designed to alleviate traffic flowing into and out of central Berlin, the U Bahn was rapidly expanded until the city was divided into East and West Berlin at the end of World War II. 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 East Berlin U Bahn lines from West Berlin were severed, except for two West Berlin lines that ran through East Berlin (U6 and U8). These were allowed to pass through East Berlin without stopping at any of the stations, which were closed. Friedrichstraße was the exception because it was used as a transfer point between U6 and the West Berlin S Bahn system, and a border crossing into East Berlin. The system was reopened completely following the fall of the Berlin Wall and German reunification. The Berlin U Bahn is the most extensive underground network in Germany.

A full GSM (GSM 900 and GSM 1800) mobile phone network for Germany's four carriers is in place throughout the U Bahn system of stations and tunnels. This system was in place by 1995 for the E Plus network, and was one of the first metro systems in the world to allow mobile telephone use; by the late 1990s the other networks could be used in some portions as well. Since 2015, UMTS and LTE is also available for E Plus and O2 (LTE since 2016) customers, and since 2020 mobile reception in some underground sections has also been extended to Deutsche Telekom and Vodafone Germany customers, with complete reception for the latter two telcos expected to be realised by mid 2021. Many of the carriages on the U Bahn feature small flat screen displays that feature news headlines from BZ, weekly weather forecasts, and ads for local businesses. Most major interchange stations have large shopping concourses with banks, supermarkets, and fast food outlets.

There are several stations, platforms and tunnels that were built in preparation for future U Bahn extensions, and others that have been abandoned following planning changes. For example, platforms have already been provided for the planned "U3" at Potsdamer Platz on the planned line to Weißensee. It is unlikely that this line, which had the working title "U3" will ever be built, so the platforms have been partially converted into a location for events and exhibitions. The line number "U3" has been used to re number the branch to Krumme Lanke, which had been part of "U1". Line D, today's U8, was intended to run directly under Dresdner Straße via Oranienplatz to Kottbusser Tor. This segment of tunnel was abandoned in favour of a slightly less direct route in order to provide the former Wertheim department store at Moritzplatz with a direct connection. This involved the construction of a 90 degree curve of the line between Moritzplatz and Kottbusser Tor stations. The construction of the tunnel under Dresdner Straße had only been partially completed before abandonment, leaving it with only one track. This tunnel is separated into three parts, as it was blocked by a concrete wall where it crossed the border between East and West Berlin. 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. Stralauer Tor was a station on the eastern bank of the Spree between Warschauer Straße and Schlesisches Tor stations. It was completely destroyed in World War II. It had been opened in 1902 and was renamed Osthafen in 1924. Today, only struts on the viaduct remain to indicate its location. In the post Second World War period it was not thought necessary to rebuild the station, due its close proximity to the Warschauer Straße station. Also its location was directly on the border between the Soviet and American sectors. 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. Nürnberger Platz station was closed on 1 July 1959. It was replaced by two new stations on either side, Augsburger Straße and an interchange station to the U9 at Spichernstraße. Today, nothing remains of the station as a third track siding was constructed in its place. Another tunnel, which once connected the U4 to its original depot and workshop at Otzenstraße (Schöneberg), is still in existence. 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. Platforms at five stations, Rathaus Steglitz, Schloßstraße, Walther Schreiber Platz, Innsbrucker Platz, and Kleistpark, were provided for the planned but never constructed U10. The U10 platform at Kleistpark has been converted into office space for the BVG. At Schloßstraße, U9 and U10 were planned to share two directional platforms at different levels; the would be U10 tracks have been abandoned, leaving both platforms used by U9 trains only. The other U10 platforms remain unused and are not generally open to the public. During the construction of Adenauerplatz (U7) station, which was built in conjunction with an underpass, platforms were also provided for a planned U1 extension from Uhlandstraße to Theodor Heuss Platz. A short tunnel section was also constructed in front of the Internationales Congress Centrum (ICC), beneath the Messedamm/Neue Kantstraße junction. This tunnel was built concurrently with a pedestrian subway and was also intended for the planned extension of the U1. The tunnel section, approximately long, ends at the location of the planned Messe station adjacent to Berlins central bus station (ZOB). The tunnel is used as a storage area for theater props. At Jungfernheide station, double U Bahn platforms similar to those at Schloßstraße were built for the planned extension of the U5. The unused platform sides are fenced off. The finished (U5) tunnel section which leads off towards Tegel airport is now used for firefighting exercises. On 4 December 2020, the U5 extension between Alexanderplatz and Brandenburger Tor was opened. This included the new Unter den Linden station, which acts as a transfer point between the lines U5 and U6. Französische Straße station on the U6 was simultaneously closed due to its short distance to the new station.

The Berlin U Bahn uses 750 volt DC electric trains that run on standard gauge (1435 ) tracks. The first trains were based on trams; they have a width of , and take their power from an upward facing third rail. To accommodate greater passenger numbers without lengthening the trains—which would require costly extended platforms—trains that ran on lines built after World War I were required to be wider. The original trains and lines, which continued to operate, were designated Kleinprofil (small profile), and the newer, wider trains and lines were designated Großprofil (large profile). Großprofil trains are wide, and take their power from a downward facing third rail. This is similar to New York City's A Division and B Division systems, where the B Division trains are wider than A Division trains (though B Division trains are also longer, while Großprofil trains are generally about the same length as Kleinprofil ones). Although the two profiles are generally incompatible, Kleinprofil trains have been modified to run on Großprofil lines during three periods of economic difficulty. Between 1923 and 1927 on the Nord Süd Bahn, and between 1961 and 1978 on the E line, adapted Kleinprofil trains were used to compensate for the lack of new Großprofil trains: they were widened with wooden boards to reach the platforms; and had their power pickups adapted to accept power from the negatively charged downward facing third rail, instead of positively charged upward facing third rail. 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. They were widened with metal boards by on each side and elevated by to close the gap to the platforms; their power pickups were designed reversible to work on both profiles. As of October 2019, IK rolling stock is still used on the U5; it is intended to move the trains to Kleinprofil lines once new Großprofil rolling stock has been delivered. As of 2007, Kleinprofil trains run on the U1, U2, U3, U4 and U5 lines; and Großprofil trains operate on the U5, U55, U6, U7, U8, and U9 routes.

Depots of the Berlin U Bahn fall into one of two classes: main workshops (Hauptwerkstätten, abbreviated as Hw); and service workshops (Betriebswerkstätten, abbreviated Bw). The main workshops are the only places where trains can be lifted from the tracks; they are used for the full inspections required every few years, and for any major work on trains. The service workshops only handle minor repairs and maintenance, such as replacing windows, or removing graffiti. As of 2005, the only dedicated Kleinprofil depot is at Grunewald (Hw Gru/Bw Gru), which opened on 21 January 1913. The first Großprofil depot opened at Seestraße (Hw See/Bw See) in 1923, to service the Nord Süd Bahn. It has 17 tracks—2 for the main workshop, and 15 for the service workshop—but its inner city location prevents any further expansion. Due to BVG budget cuts, the Seestraße depot also services Kleinprofil trains. Two further Großprofil service workshops are located at Friedrichsfelde (Bw Fri), and Britz Süd (Bw Britz). In the past, there were other workshops. The first opened in 1901 at Warschauer Brücke, and was the construction site for most of the early U Bahn trains. The division of the U Bahn network on 13 August 1961 forced its closure, although it was reopened in 1995 as a storage depot. A small depot operated at Krumme Lanke between 22 December 1929 and 1 May 1968; and, while the network was split, East Berlin's U Bahn used the S Bahn depot at Schöneweide, along with a small service workshop at Rosa Luxemburg Platz, which was closed following reunification.

The Berlin U Bahn ranks among the safest modes of transport: its history features few accidents. 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 26 September 1908, a train driver missed a stop signal. As a result, two trains collided at the junction, and one fell off the viaduct. The accident killed eighteen people, and severely injured another twenty one. Gleisdreieck's triangular layout had already been deemed unsuitable for future developments; this incident—and a later, less serious one—triggered its reconstruction as a multi level station, starting in 1912. On 30 June 1965, a train with brake failure stopped on the G line—today's U9—between Zoologischer Garten and Hansaplatz. Unaware of the faulty train, a mechanic working at the Zoologischer Garten signal tower noticed that the signal for the affected section had been set to "Stop" for a long time. Thinking it was a fault of his, after several attempts he manually overrode the signal, in defiance of regulations that strictly prohibited such actions. The following train, which had been waiting at Zoologischer Garten, then left the station on the same track. With emergency brakes unable to prevent the accident, the two trains collided. One passenger was killed in the crash, and 97 were injured. The mechanic was fined 600,000 DM. Fires can be particularly dangerous and damaging within an underground system. In October 1972, two trains and a length of tunnel were completely destroyed when the trains caught fire; the reconstructed tunnel is clearly distinguishable from the old one. Another train burned out in the connecting tunnel between Klosterstraße and Alexanderplatz in 1987. 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. The single exit of the station was unreachable, forcing the passengers to run through the tunnel to reach the next emergency exit. The fire also damaged the station, which remained closed until that September. 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. Installation of Portugal's gift to the city was completed on 30 October 2002. As a consequence of the Deutsche Oper incident, BVG decided to post an employee at every station with only one exit until a second exit could be built. Over the following few years, many of those stations—including Britz Süd, Schillingstraße, Viktoria Luise Platz, Uhlandstraße, and Theodor Heuss Platz—were retrofitted with additional exits. By June 2008, the only remaining stations with no second exit, Konstanzer Straße and Rudow, had been fitted with second exits. Despite these changes, several passenger organisations—such as Pro Bahn, and IGEB—demand that stations with exits in the middle of the platform also be fitted with additional emergency exits. Many stations are built this way; meeting those demands would place a heavy financial burden on both the BVG and the city. The U6 saw a particularly costly, though casualty free, incident on 25 March 2003. Scheduled repair work on the line limited the normal service to between Alt Mariendorf and Kurt Schumacher Platz; one train then shuttled back and forth between Kurt Schumacher Platz and Holzhauser Straße, sharing a platform at Kurt Schumacher Platz with the normal service trains departing for their return journey to Alt Mariendorf. Needing to pass several stop signals on the shuttle service, the driver had been given special instructions how to proceed. Unfortunately, he ignored the signal at the entry to Kurt Schumacher Platz, and ploughed into the side of a train heading back to Alt Mariendorf. The impact wrecked both trains, and caused considerable damage to the tracks. Normal service did not resume for two days, and the removal of the two wrecked trains—which, surprisingly, could still roll along the tracks—also took nearly 48 hours.

The Berlin U Bahn has appeared in numerous films and music videos. Offering access to stations, tunnels, and trains, the BVG cooperates with film makers, although a permit is required. 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! You're Dead! (1987 film) (1987) featuring Ingolf Lück, Run Lola Run (1998), and several Tatort episodes. The previously unused Reichstag station was used to shoot scenes of the movies Resident Evil and Equilibrium. The U Bahn station Messe was used as coverage in the films Hanna and The Hunger Games: Mockingjay – Part 2. Möbius 17, by Frank Esher Lämmer and Jo Preussler from Berlin, tells the story of an U Bahn train that, caught in a Möbius strip, travels through alternate universes after a new line is built. 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. The film's scenes feature a recreation of the station as it was in 1928—rather darker and dirtier than in the 21st century. In the surrealistic two hour epilogue, Fassbinder transforms parts of the station into a slaughterhouse where people are killed and dissected. Since 2001, the Berlin U Bahn has hosted the annual short film festival Going Underground. Short films (up to 90 seconds long) are shown on the monitors found in many of the U Bahn trains. Passengers on board vote for the festival winner. 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. "Linie 1", a musical performed by Berlin's Grips Theater, is set completely in stations and trains of the Berlin U Bahn; a movie version has also been produced. In 2002, the BVG cooperated with design students in a project to create underwear with an U Bahn theme, which, in English, they named "Underwear". 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). After the first series sold out quickly, several others were commissioned, such as Nothammer (emergency hammer), and Pendelverkehr (shuttle service; though Verkehr also means "intercourse" and Pendel also means "pendulum"). They were withdrawn from sale in 2004.