Rhein Ruhr Express is a regional rail system in the Rhine-Ruhr metropolitan region of Germany connecting the Ruhr Valley and the central Rhineland. The primary goal is to provide a fast connection on the main trunk line between Dortmund and Cologne, located along the Ruhr Valley, with trains every quarter of an hour. This main line has been expanded, improved or reconfigured to enable more trains to operate and at higher speeds. Like the previous Regional-Express lines they replaced, RRX routes branch off from the main trunk line to serve other cities and states at lower frequencies.

The Siemens Desiro is a family of diesel or electric multiple unit passenger trains developed by Siemens Mobility, a division of the German Siemens AG conglomerate. The main variants are the Desiro Classic, Desiro ML, Desiro UK and the later Desiro City, Desiro HC and Desiro RUS. The trains are mostly used for commuter and regional services, and their rapid acceleration makes them suitable for services with short distances between stations. The design is flexible, and has become common in many European countries.