The Ruhr–Sieg railway is a 106 km long double-track, electrified main line from Hagen to Siegen via Iserlohn-Letmathe, Finnentrop and Kreuztal in the German state of North Rhine-Westphalia.

The committees in both Siegen and Hagen selected a more easterly route through the Hundem valley because it would be shorter and therefore cheaper.

The highest point of the railway line between Hagen and Haiger is Welschen Ennest at almost 411 m above sea level.

Since narrow valleys had to be crossed, the Borsig company built a special locomotive, class 675, designed for its curves.

Tudor style stations were built in Plettenberg, Grevenbrück and Kreuztal, attracting attention as unusual designs for the region (today they are listed monuments).

As late as 1980, express trains ran from Munich via Stuttgart and Frankfurt, over the Ruhr–Sieg line to Norddeich Mole.

Traditional stops for express trains were Hagen, Letmathe, Altena, Finnentrop, Lennestadt-Altenhundem, Kreuztal and Hüttental-Weidenau (now called Siegen-Weidenau).

Regional-Express and Regionalbahn trains from Iserlohn and Siegen are coupled together in Iserlohn-Letmathe station, and continue as a unit via Hagen, Witten and Bochum to Essen.

In the opposite direction, trains are uncoupled in Iserlohn-Letmathe station and then run separately to Siegen and Iserlohn.

Further connecting curves and junctions in Hagen-Kabel and Hohensyburg allow the transport of freight between Gießen and the Ruhr region without reversal and bypassing Hagen Hauptbahnhof.

In the last few years, almost all stations from Altena to Siegen-Weidenau have been modernised under a program of the state of North Rhine-Westphalia, including improvements of access for the disabled.