The Wupper-Express (RE 4) is a Regional-Express service in the German state of North Rhine-Westphalia (NRW) running from Aachen via Mönchengladbach, Düsseldorf, Wuppertal, Hagen to Dortmund. The service is operated every hour by DB Regio NRW. It is the third most widely used Regional-Express line in the area administered by the Verkehrsverbund Rhein-Ruhr with approximately 24,000 passengers a day.

Today's RE 4 is the successor to the former StädteExpress line SE from Aachen to Hagen and Iserlohn. Later, the end point was moved to Hamm and after the abolition of InterRegio services it was extended to Munster. Under the second stage of North Rhine-Westphalia’s integrated timetable (ITF 2), introduced in December 2002, it was replaced by the Maas-Wupper-Express (RE 13) and the Ems-Börde-Bahn (RB 89) services between Hagen and Munster and the Wupper-Express has since then run to Dortmund with a stop in Witten.

The Wupper-Express runs successively over the Aachen–Mönchengladbach, the Mönchengladbach–Düsseldorf and the Düsseldorf–Elberfeld lines. The service then follows the Elberfeld–Dortmund railway as far as Witten station, from where it uses the tracks of the Witten/Dortmund–Oberhausen/Duisburg railway and the Oberstraße Tunnel on its way to Dortmund station. At night, the RE 4 operates to Düsseldorf Airport Terminal station.

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