same machinery as for the building of the new tunnel of the Elbe tunnel. Services Services operate with a 5 or 10-minute frequency with the exception of the northeastern peripheral branches of U1 that have a 10 or 20-minute frequency. During peak times the frequency is increased to 3/3/4-minute or even 2/3-minute patterns. Since the timetable change in December 2004 the U-Bahn operates an all-night, 20-minute service interval on Friday and Saturday nights. From Sunday to Thursday the U-Bahn operates from 4 am to about 1 am. Stations Hamburg Hauptbahnhof Hamburg Hauptbahnhof is a central junction with the mainline railway, and, due to the confluence of all U-Bahn lines and all S-Bahn lines, an important interchange for suburban public transport as well. Tunnels for the first underground railway were constructed at the same time as the Hauptbahnhof itself. The tunnels run underneath the mainline tracks, which are themselves below street level. The current U3 stops at this station, which, for more clarity, is now known as Hauptbahnhof Süd. There is also an underground pedestrian tunnel parallel to the U3 circle line tunnel, which allowed a quick connection to the mainline platforms until the major part of it was closed off in 1991. The remaining part of the tunnel can still be used today to reach the S-Bahn platforms. The unusual width of the vaulted station is due to the fact that it originally housed four tracks; the Rothenburgsort branch, which was destroyed in World War II and never rebuilt, used to begin here. The inner tracks remained in service until the 1970s and were used to terminate additional service trains until the trackbeds were covered to create one wide instead of two narrow platforms. The tracks that led up the ramp to the decommissioned line were used as layup tracks during that time. In 1960 the tunnel leading to the ramp was cut off as the then-built U1 tunnel crosses right through it. The tunnels for the U1 were driven under the mainline railway next to the U3 tunnels in 1959, the platform for the U1 lies directly south to the U3 and is accessible via the same station entrances. The opening of the line U2 (U21 in 1969) lead to the construction of a further underground station Hauptbahnhof Nord to serve it. The station is below the surface; in the direction of the Jungfernstieg station, the tunnels pass under