The Rhine-Main Transport Association (Rhein-Main-Verkehrsverbund) has given unique numbers to its regional railway services in regular operation. Rhine-Main S-Bahn lines have a single digit with an "S" prefix, as is standard in Germany, while the remaining lines have two-digit numbers, preceded by two letters indicating the train class (Regionalbahn (RB), Regional-Express (RE) and Stadt-Express (SE)). The lines are mainly run to and from Frankfurt as the metropolis of the Rhine-Main region, but there are also a number of connecting lines between the major towns of the whole area.
It is notable that Deutsche Bahn does not operate Stadt-Express services anymore, but the Rhine-Main Transport Association continues to offer such services. Operationally, these services run mostly as a Regionalbahn (stopping), but with part of the journey run as a Regional-Express service, especially where there are parallel S-Bahn services.