The Hamburg U-Bahn is a rapid transit system serving the cities of Hamburg, Norderstedt and Ahrensburg in Germany. Although referred to by the term U-Bahn (the "U" commonly being understood as standing for "underground"), most of the system's track length is above ground. The network is interconnected with the city's S-Bahn system, which also has underground sections. It is operated by Hamburger Hochbahn within the Hamburger Verkehrsverbund (HVV). It was opened in February 1912, and comprises four lines serving 93 stations, with a route length of in 2019. History In 1906 the Senate of Hamburg awarded a contract for the Elevated and Underground Railway to Siemens & Halske and AEG of Berlin. The first stretch was completed on 7 October 1906. This was followed in 1911 with the founding of the Hamburger Hochbahn Aktiengesellschaft (HHA). Thus Hamburg became the third German city (after Berlin, 1902 and Schöneberg, 1910) to have a U-Bahn (then known as the Elevated and Underground Railway (Hoch- und Untergrundbahn) as it is largely either elevated or in tunnels – not much at street level). The construction of such a railway had long been discussed, meanwhile an elevated train was also considered. 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 und Barmbek was inaugurated. The stretch included both underground and elevated sections. Network development By 1915 the Ringbahn and the three spurs, from Kellinghusenstraße to Ohlsdorf, from Schlump to Hellkamp (today: disused station between Oster- and Lutterothstraße) and from central station to Rothenburgsort were completed. By 1934 the extensions from Ohlsdorf to Ochsenzoll (Langenhorner Bahn), from Kellinghusenstraße to Jungfernstieg (KellJung line), and the Walddörferbahn (Forestvillages line) from Barmbek to Großhansdorf and Ohlstedt were completed. In 1943, during the Second World War the subway was badly damaged during 'Operation Gomorrha,' the RAF bombing of Hamburg that summer. The stretch from Hauptbahnhof to Rothenburgsort was not repaired because the district it passed through had been completely destroyed. It was five more years before the remaining network was once more completely intact. Further additions to the network were made, starting in 1960. The KellJung line was extended via Meßberg to the