stock are not compatible with each other. The first DC trains, of the type ET 171, were delivered in 1939; daily service began in July 1940 alongside the AC trains. Due to the Second World War and the post-war years with their shortages, mixed usage lasted until 1955. The basic DC train unit consisted of three four-axle carriages, each with four sliding double doors per side. The middle carriages had upholstered seats for second class, while the motorised end carriages had third-class wooden seats. Network extensions from 1950 to 1965 The DC S-Bahn was extended along the single-track suburban line from Blankenese to Sülldorf in 1950 and to Wedel in 1954. A section of the mainline railway between Hamburg and Berlin, which, due to the division of Germany, had very little traffic, between Haupbahnhof and Bergedorf was added to the network in 1959 by the addition of the third rail. This was the first section where S-Bahn and mainline trains (the number of the latter remained small until 1990) shared tracks. It was the second S-Bahn line, from Bergedorf via Berliner Tor to Altona. In 1962 a connecting curve was constructed from the Verbindungsbahn at Holstenstraße to the Altona-Kaltenkirchen railway, whose terminus was relocated to Langenfelde. The S-Bahn was extended in 1965 along the AKN to Eidelstedt and from there along the mainline tracks towards Kiel as far as Elbgaustraße. HVV and a line numbering system In 1965, the German Railway, along with two local transport companies, founded the Hamburger Verkehrsverbund, a common tariff system for U-Bahn and bus lines. The S-Bahn joined in December 1966. From January 1967, the S-Bahn lines were designated S1−S6 (see Network). Line designations with a leading letter "S" have since been adopted for other S-Bahn systems in the German-speaking world. Analogically, U-Bahn lines got the leading letter "U". Network extension after 1967 Extensions were made in 1967 on separate tracks from Elbgaustraße to Pinneberg, and in 1969 on existing mainline tracks from Bergedorf to Aumühle. To ease the strain on the central Verbindungsbahn ("connection railway"), a second main connection was constructed, the City S-Bahn, which traverses the city centre in a tunnel. The first section was opened in 1975 between Hauptbahnhof and Landungsbrücken, the extension to Altona in 1979 and was completed in 1981 with the connection above ground to Diebsteich. 1983 saw the opening of the line via Wilhelmsburg to Harburg Rathaus, a