For comparison, the Wisser Valley Railway (Wissertalbahn) between Wissen and Morsbach generated only 6,800 marks per km.

In addition, during the two world wars and the occupation of the Ruhr,[7] the line together with the Wiehl Valley Railway was historically significant as a diversion route for coal trains[8] and other important traffic.

This was also one of the reasons for the closure of passenger services, partly caused by poor design, including the lack of a direct connection from Olpe to Gummersbach except via an awkward set of points in the Dieringhausen district.

[11] The planners of the Siegburg–Olpe railway preferred a direct connection to Cologne from the beginning rather than the indirect route via Siegburg.

[12] Another early plan proposed a line through the Agger valley from Siegburg to the Ruhr as part of the East Rhine Railway.

Before its construction, some critics claimed that its operations would not be profitable, so it was only agreed after long discussions and after the exertion of political pressure.

The critics came mainly from the Cologne area because of its connection to Siegburg, as well as from the staff of the Bergisch-Märkische Railway Company, which favoured a route to the north.

Therefore, the population enthusiastically celebrated its completion for several hours, despite the long connecting times to Siegburg and Cologne.

[19] In 1910, the still operating section between Overath and Cologne was completed, ending through services between Siegburg and the Oberbergische district.

The travel time from the Bergisches Land to Cologne was shortened to one and a half hours by the elimination of the detour via Siegburg and the old Siegburg–Overath route lost its importance.

[20] Between 1910 and 1914, the line was relaid in many places on the hillside, as there had been many fatal traffic accidents and complaints of noise pollution in the area between Osberghausen and Derschlag.

This 6.9 km long line was approved in March 1914, but the outbreak of the First World War, prevented its construction.

[24] The high reparations required by the Versailles Treaty led to a shortage of rolling stock and a limited timetable.

Due to the large amounts of heavy trains that ran on the line to the east, the cheaply built superstructure was damaged.

[25] During the construction of Agger Valley Dam (Aggertalsperre) there was a high volume of commuters by immigrant workers to Derschlag, changing to the Gummersbach Light Railways to continue through Dümmlinghausen to Genkelmündung.

[26] During World War II, mobile guns were installed on the line in Hützemert because the east-west link again played an important role as a diversion route.

[28] The diversion route stopped because of destruction to the west by German troops under Hitler’s scorched earth policy.

[30] The Deutsche Bundesbahn railway division (Bundesbahndirektion) at Wuppertal informed locals from 1950 of the proposed closure of the western section, which took place after the thinning of services in 1954.

[33] Hopes for a resumption of passenger service between Siegburg and Overath were dashed in 1959 with the issue of a license to close the line.

The remaining part of the line between Siegburg and Lohmar was transferred to the jurisdiction of the railway division at Cologne.

[34] The last regular passenger service operated between Olpe and Dieringhausen on 28 December 1979 despite protests from the public and local politicians.

An embankment collapsed[36] due to poor construction near Wiedenest on 27 May 1989 and freight operations from Bergneustadt to Drolshagen were abandoned and the section was closed.