The S5 service of the S-Bahn Rhein-Main system bearing the KBS (German scheduled railway route) number 645.5 The Homburg Railway connects Frankfurt Central Station with Friedrichsdorf, via Bad Homburg.

S-Bahn services commenced on the line on 25 September 1977.

In a prior test operation it ran between Friedrichsdorf and Frankfurt Central Station.

After the opening of the Frankfurt Citytunnel the service was renamed to S5 and extended to the new Hauptwache underground station.

Further extensions of the tunnel followed in 1983 (Konstablerwache) and 1990 (Ostendstraße and Lokalbahnhof) so that the Südbahnhof (South station) became the service's eastern terminal.