The Hamburg S-Bahn is a suburban commuter railway network in the Hamburg Metropolitan Region. Together, the S-Bahn, the Hamburg U-Bahn, the AKN railway and the regional railway form the backbone of railway public transport in the city and the surrounding area. The network has operated since 1907 as a commuter rail system, under the direction of the state railway, and is a member of the Hamburger Verkehrsverbund (HVV; Hamburg Transport Association). There are six lines, serving 68 stations, on of route. On an average working day the S-Bahn transports about 590,000 passengers; in 2010 about 221 million people used the S-Bahn. The S-Bahn is the only railway in Germany that uses both 1,200 V DC supplied by a third rail and supplied by overhead lines. Most of the tracks are separated from other rail services. The S-Bahn is operated by S-Bahn Hamburg GmbH, a subsidiary of DB Regio. Similarly to Berlin but unlike Hanover, the S-Bahn is an important part of public transport within the city due to its dense schedule and good coverage of the metropolitan region. Unlike both Berlin and Hanover, the S-Bahn is of little importance for regional traffic since the network lies mostly within the city, though in 2007 the southwestern S3 line was extended about into the state of Lower Saxony (the Neugraben - Stade portion, which included seven new stations). History 1906: Opening On 5 December 1906, under the description , the Prussian Eisenbahndirektion (railway division) of Altona opened with steam trains between Blankenese, Altona (Elbe) and Hamburg. The Stadt- und Vorortbahn (City and Suburban railway) included the Altona-Blankenese line (Altona-Blankeneser Bahn, opened in 1867), the local tracks of the Hamburg-Altona link line (Verbindungsbahn, opened in 1866) and a new section to Ohlsdorf. The Verbindungsbahn had been extended from one track to four and level crossings eliminated between 1893 and 1903. The new double-track line adjoining it was completed in the summer of 1906 after an eight-year construction period. It ran alongside the Lübeck–Hamburg line of the Lübeck-Büchen Railway Company as far as Hasselbrook and then on its own tracks as far as the new Ohlsdorf cemetery. A new main cemetery with good transit connections was necessary in part due to the extension of the central railway lines, which had reduced the area of the existing already strained cemeteries near the city's medieval fortifications. 1907/08: First electric operation The line was electrified with overhead