The Rhein Weser Express (RE 6) is a Regional-Express service route in the German state of North Rhine-Westphalia, connecting some of the most important cities in Westphalia (among others Minden, Bielefeld and Hamm) with the Ruhr (especially Dortmund, Bochum, Essen, Duisburg, Düsseldorf, Neuss and Cologne).

The reason for the temporary separation of RE 6 and RE 6a was the lack of emergency brake connections between the double-deck cars used on line RE 6 (Minden–Düsseldorf), which were not allowed to run through the Cologne Airport loop.

At the change of timetable in December 2016, the rolling stock was replaced by retrofitted double-decker trains, the RE 6 was extended to Cologne/Bonn Airport and the line RE 6a service was discontinued.

Since the timetable change in December 2019, the Rhein-Weser-Express has been operated by National Express with Siemens Desiro HC EMUs.

The Rhein-Weser-Express is linked with the rest of the transport network in North Rhine-Westphalia and Lower Saxony in Cologne, Düsseldorf, Duisburg, Essen, Dortmund, Hamm, Bielefeld, Herford, Löhne and Minden.

The "large traffic" contract (große Verkehrsvertrag) that covers the route between Dortmund and Düsseldorf was terminated by the VRR on 12 June 2008 without notice, since then DB has operated this branch under direction.