The Rhein-Sieg-Express is a Regional-Express service in the German state of North Rhine-Westphalia and Rhineland-Palatinate running from Aachen via Düren, Cologne, Troisdorf, Siegburg and Betzdorf to Siegen.

Regional Express (RE) line 9 has existed since the introduction of the integrated timetable (German: integraler Taktfahrplan, ITF) in North Rhine-Westphalia in 1998.

From 1998 the train service ran on the route from Krefeld via Neuss, Cologne, Siegburg and Siegen to Gießen.

The causes of these delays were the single track sections of the Sieg Railway, work in connection with the completion of the Cologne–Frankfurt high-speed line and the upgrading of the Sieg Railway for S-Bahn operations, speed restrictions between Cologne and Krefeld and a very short turnaround time in Krefeld of 12 minutes.

[3] Therefore, services are to be operated with a total of 15 electric multiple units of the E-Talent 2 variety of the Bombardier Talent called class 442.

These additional trains run between Au (Sieg) and Siegen with stops in Etzbach, Niederhövels, Scheuerfeld and Brachbach.