The Rhine-Main S-Bahn system is an integrated rapid transit and commuter train system for the Frankfurt/Rhine-Main region, which includes the cities Frankfurt am Main, Wiesbaden, Mainz, Offenbach am Main, Hanau and Darmstadt. The network comprises nine S-Bahn lines, eight of which currently travel through the cornerstone of the system, a tunnel through central Frankfurt. The first section of this tunnel was opened on May 28, 1978. Further tunnel sections were opened in 1983 and 1990, before its completion in 1992. The system belongs to the Rhein-Main-Verkehrsverbund (RMV) and is operated by DB Regio, a subsidiary of Deutsche Bahn.

Wiesbaden is the capital of the German state of Hesse, and the second-largest Hessian city after Frankfurt am Main. With around 283,000 inhabitants, it is Germany's 24th-largest city. Wiesbaden forms a conurbation with a population of around 500,000 with the neighbouring city of Mainz. This conurbation is in turn embedded in the Rhine-Main Metropolitan Region—Germany's second-largest metropolitan region after Rhine-Ruhr—which also includes the nearby cities of Frankfurt am Main, Darmstadt, Offenbach am Main, and Hanau, and has a combined population exceeding 5.8 million.