The S9 service of the S-Bahn Rhein-Main system bearing the KBS (German scheduled railway route) number 645.9. It is largely concurrent with the S8 service, diverging only to bypass Mainz (which the S8 serves locally).
The city tunnel is an underground, pure S-Bahn route used by almost all services (except for the S7 service which terminates at the central station). In a short section between Mühlberg and Offenbach-Kaiserlei a line parallel with the South Main railway is used.
This service was first introduced in 2000 to provide an alternative route to Wiesbaden via Frankfurt Airport skipping Rhineland-Palatinate and its capital Mainz.
