The City S-Bahn is a 5,380 metre-long tunnel section of the Hamburg S-Bahn. It runs between Altona station and Hamburg Hauptbahnhof through the city centre of Hamburg and the districts of Altona and St. Pauli. The S-Bahn lines S1, S2 and S3 of the Hamburger Verkehrsverbund (Hamburg Transport Association) run through the tunnel. Seven S-Bahn stations are located in the tunnel, including the tourist-oriented stations of Landungsbrücken and Reeperbahn. Route The tunnel begins directly after the Hauptbahnhof where its two tracks branch off the Hamburg-Altona link line and immediately dive under it. The ramp has a maximum gradient of 3.94%, since only a few hundred metres from the beginning of the tunnel it begins to run under the Inner Alster Lake to Jungfernstieg station, which is built under the lake. The line runs from there to the southwest, passing through Stadthausbrücke station to Landungsbrücken station. From there, it swings back to the northwest and tunnels under the Reeperbahn to Reeperbahn station. It then continues to Königstrasse station and reaches Altona station from the south. The underground S-Bahn station at Altona was built with four tracks, including a reversing facility. Shortly north of the station, the railway climbs at a maximum gradient of 4.0% to reach ground level. History In the 1960s, Hamburg had only two S-Bahn lines. During the planning and construction of the large housing estate of Osdorfer Born in north-western Hamburg, consideration was given to increasing S-Bahn services on the Hamburg-Altona link line (Verbindungsbahn), which at that time had services running at a minimum interval of 150 seconds. The signalling system allowed trains to run 90 seconds apart but the slightest deviation from the timetable would have created a major difficulty. Moreover, the link line avoided the entire central city of Hamburg and the former city of Altona, as it ran along part of the former Hamburg ramparts. As a result, it was decided to build an underground S-Bahn line to the south of the Link line to open up the inner city—the City S-Bahn line. Construction, which was planned to take14 years, started on 14 October 1967. Building the line proved to be extremely difficult, since in addition to passing under the Inner Alster Lake, the line was almost completely in tunnel and does not always run under existing roads, instead cutting across some blocks. A particular challenge to the engineers was the Jungfernstieg station, where the