The Siegburg–Olpe railway or Agger Valley Railway ( or Aggertalbahn) is a single-tracked, non-electrified branch line in the German state of North Rhine-Westphalia. Part of a direct link from Cologne, only the section from Overath to Gummersbach-Dieringhausen is still in operation. The section of line from Siegburg to Overath and from Dieringhausen to Olpe are closed. The line is named after the River Agger, which it follows for a long way and crosses several times. Significance Although the line as always been considered by its operator as a branch line, it is central to the railway network of the Oberbergisches Land and it has been called a "secret" main line. Like the other lines in the region it primarily served local industry (mainly the textile and metal industries). Unlike, for example, the Wiehl Valley Railway (Wiehltalbahn), the line was not primary built to serve the quarry industry, it also used to connect to company sidings and to other branch lines. Passenger played only a secondary role from the beginning. Despite the fears of critics, the line was initially very profitable: the line from Siegburg to Derschlag was the most profitable branch line between Cologne and Kassel. This section was opened in 1896 while its continuation to Bergneustadt was still being planned. Accounts from 1892/1893 show that it had revenues of 21,000 marks per kilometre. For comparison, the Wisser Valley Railway (Wissertalbahn) between Wissen and Morsbach generated only 6,800 marks per km. The following standard gauge branch lines branched from the Agger Valley Railway: the Cologne–Overath railway as a direct connection from the Oberbergisch district to Cologne the Wiehl Valley Railway to Osberghausen the Volme Valley Railway (Volmetalbahn) to Dieringhausen the tramlines and freight sidings of the Gummersbach Light Railway (Gummersbacher Kleinbahnen) to Niedersessmar and Derschlag the Finnentrop–Freudenberg railway to Olpe. Up to the 1950s, there was also in Engelskirchen a loading bay of the metre-gauge Leppe Valley Railway (Leppetalbahn) to Marienheide and a very small narrow-gauge railway to Drolshagen. In addition, during the two world wars and the occupation of the Ruhr, the line together with the Wiehl Valley Railway was historically significant as a diversion route for coal trains and other important traffic. In the 1970s it was used as a scenic railway. Until the 1970s there were many sidings serving industrial companies such as Delta in Derschlag and Dörrenberg Edelstahl in Ründeroth. The traffic on the now closed