Wikipedia
Stadt-Express
The Stadt-Express (SE), formerly City-Bahn (CB), is a train category in Germany, that links conurbations with the outer reaches of the surrounding countryside. The name literally means "City Express".
While Deutsche Bahn no longer offers Stadt-Express services, the Rhein-Main-Verkehrsverbund tariff association still orders them. As a consequence, there are Stadt-Express lines in the RMV-area, but they are categorized by the Deutsche Bahn as either Regionalbahn or Regional-Express.
Except for the RMV-area, the Stadt-Express lines were everywhere else either renamed to Regionalbahn or Regional-Express or were discontinued.