Although referred to by the term U-Bahn (the "U" commonly being understood as standing for Untergrund "underground"), most of the system's track length is above ground.

It was opened in February 1912,[4] and comprises four lines serving 93 stations,[2] with a route length of 106.4 kilometres (66.1 mi) in 2019.

[6] In 1906 the Senate of Hamburg awarded a contract for the Elevated and Underground Railway to Siemens & Halske and AEG of Berlin.

In 1906 a circular route was begun, connecting Hamburg Hauptbahnhof – Berliner Tor – Barmbek (earlier spelling: Barmbeck) – Kellinghusenstraße – Schlump – Landungsbrücken (earlier designation: Hafentor) – Rathaus – Hauptbahnhof with branches to the quarters of Eimsbüttel, Ohlsdorf and Rothenburgsort.

On 15 February 1912 the first stretch of the future Ringbahn (Circle railway) between Rathaus station and Barmbek was inaugurated.

In 1943, during the Second World War the subway was badly damaged during 'Operation Gomorrha,' the RAF bombing of Hamburg that summer.

A new core stretch of the line U2, between Schlump and Berliner Tor, became a direct connection through the city centre.

The line extension opened in 1996 between Garstedt and Norderstedt Mitte, which replaced the southern part of the Alsternordbahn.

In order to realise a problem-free service, a redesign of the current track layout to the west of Berliner Tor was necessary.

An originally planned grade separated crossing of the two lines was realised, which was left undone before due to cost.

Trains coming from the city centre leave the ring at Barmbek and continue via the viaduct stretch to Wandsbek Gartenstadt.

[9][10] The eastern branch of the "Walddörferbahn" was originally intended to terminate at the station Beimoor, just to the north of Großhansdorf, in order to connect a proposed housing development to the public transport system.

Today the remains of the station are used as a winter home for bats in the middle of a dry grass biotope.

Until the connection with the rest of the network was ready, it was intended to transport trains to the main service depot in Barmbek by flatbed trucks for maintenance.

Platforms and track beds for this stretch were completed in the stations Hauptbahnhof Nord and Jungfernstieg.

Preparations for tunnel constructions were made in the area of Hamburg-Altona railway station and under the motorway A 7 in Bahrenfeld.

The northern surplus platform area at Hauptbahnhof Nord is currently used for the art installation Sterne by Raimund Kummer and Stefan Huber.

Today both unused platforms appear deteriorated and large segments of the wall coverings are missing, which is because the complete station already suffered from water damage in the 1970s (and, to a smaller extent, still frequently does so today) and it was necessary to remove those segments in order to fix the leaking ceiling and walls.

They include Original plans also called for the U4 to be extended southward to the S-Bahn station Harburg Rathaus via Wilhelmsburg, in order to provide a second rapid transit line to serve the south of Hamburg and relieve the busy S3/S5 that is - since its opening in 1983 - the only rapid transit service for the complete borough of Harburg and nowadays operates at its capacity limit.

There had been an ongoing discussion about the costs and the benefit of this line, but the government of Hamburg decided to build it nevertheless.

Since the timetable change in December 2004 the U-Bahn operates an all-night, 20-minute service interval on Friday and Saturday nights.

The unusual width of the vaulted station is due to the fact that it originally housed four tracks; the Rothenburgsort branch, which was destroyed in World War II and never rebuilt, used to begin here.

The opening of the line U2 (U21 in 1969) lead to the construction of a further underground station Hauptbahnhof Nord to serve it.

The two outer platforms remain unused since their construction and were originally intended for a proposed U-Bahn line between Altona/Lurup and Winterhude/City Nord (see above).

The Hamburg U-Bahn uses standard gauge electric multiple units that run on third rail with 750 volts DC.

They receive small propulsion modifications, the interior redesign of 2008 (most of those units were not refurbished back then), improved departure signals and structural issues of the wearing car bodies are being fixed.

The Type DT4 came with a more futuristic design, reduced noise, increased safety and greater economy than the older trains.

Due to the long construction span the batches slightly differ from each other in terms of interior and exterior design, propulsion and destination signs.

After the experiences with the heavy DT1 trains they were constructed as light-weight cars with weaker engines to save energy.