railway under construction via Barmbek to Ohlsdorf, with freight stations being built at the two stations, and electrification of the entire Stadtbahn (City railway) between Blankenese, Altona, Hauptbahnhof (central station) and Ohlsdorf. Electrification was decided on by the Prussian State Railways as track operator, after it had already carried out extensive tests on the Schöneweide–Spindlersfeld line in Berlin, using overhead lines with 6.3 kV 25 Hz AC, the system which it selected for Hamburg. In late 1906 work was completed. The urban and suburban line to Ohlsdorf was put into operation on 5 December 1906; one day later the new Hamburg Hauptbahnhof was commissioned. About a year later operations with electric trains started on the Stadtbahn and all suburban railways, making the Hamburg network the first electrified suburban rail network in Germany. Conversion to DC and expansion of the S-Bahn AC operations continued until the late 1930s with few service problems. However, in the meantime electric operations had been established on the Berlin S-Bahn using direct current. Since the technology in use in Hamburg had to be replaced anyway it was decided to switch to using third rail DC current. Even before this decision was made the Stadtbahn and suburban railways were designated as the S-Bahn in 1934, four years after the Berlin network was so named. Unlike the 800 volt power used in Berlin, the voltage chosen in Hamburg was 1200 volts, which was technically, the maximum voltage that could be used in rail operations. Similarly, it was decided to use bottom contact third rail, which had a clear advantage at points, as the third-rail power would not have to be interrupted. In 1940 transition to the new system began, but, due to the war, conversion could not be completed until 1955. From the 1960s, there was continuous expansion of the suburban rail network. Although the link line as the core of the S-Bahn had good passenger traffic, it only touches the city centre at its northern edge. Thus, a new central underground line (subsequently known as the City S-Bahn) was planned through the central area, which had more than 300,000 jobs. This required some alterations for the old main line, to allow the connection of the new underground line to the main line at the Hauptbahnhof and its reconnection to Altona station from the south. The entire Altona station building was demolished and subsequently rebuilt in a plain