was opened, with all lines sharing the tunnel between Hauptbahnhof and Hauptwache. The initial system, which lay entirely to the north of the river Main, comprised the following lines: S1: Wiesbaden Hbf – Höchst – Hauptbahnhof – Hauptwache S2: Niedernhausen – Höchst – Hauptbahnhof – Hauptwache S3: Frankfurt-Höchst – Bad Soden – Frankfurt West – Hauptbahnhof – Hauptwache S4: Kronberg – Frankfurt West – Hauptbahnhof – Hauptwache S5: Friedrichsdorf – Frankfurt West – Hauptbahnhof – Hauptwache S6: Friedberg – Frankfurt West – Hauptbahnhof – Hauptwache In 1980, two further lines were added to the network, made possible by construction of a new rail bridge over the river Main: S14: Wiesbaden – Mainz – Flughafen – Hauptbahnhof (main arrivals hall) S15: Flughafen – Hauptbahnhof – Hauptwache Completion in 1983 of a 600m long easterly extension of the Citytunnel, as far as Konstablerwache, improved the opportunities for train turnaround in the tunnel. At this stage lines S1–S6 and line S14 were extended to Konstablerwache, while the S15 was rerouted to the main arrivals hall of the Hauptbahnhof. Later Developments In 1990 the Citytunnel was extended, adding the underground stations Ostendstrasse and Lokalbahnhof to the system, along with the overground station Frankfurt Süd. All the lines (with the exception of the S15) were extended to Frankfurt Süd, while the S5 and S6 were further extended a short time later to a new station at Stresemannallee, south-west of Frankfurt Süd. In 1992, S1 and S2 trains were diverted to the new Mühlberg underground station, the first station in the direction of Offenbach. This left the S3, S4, S5 and S6 serving all the stations between Hauptbahnhof and Frankfurt Süd, a situation which still pertains to this day. The S14, which is now the S8, also served all of these stations. In 1995, the newly instituted RMV increased train frequency from a 20/40/60 minute schedule (depending on the time of day) to the 15/30/45/60 minute schedule which is still used in the system. The S15 ceased operations at this stage. Also in 1995, a new underground alignment through the city of Offenbach was opened, assisting the rerouting of the S14 (renamed the S8) through Mühlberg to City-Tunnel Offenbach and Hanau. The S1 was also extended as far as Offenbach, while the S2 returned to serving Frankfurt Süd. In 1997, the routes of the S5 and S6 were shortened slightly, so that they travelled only as