From the time of the building of a line, a direct connection was proposed between Siegen and Haiger, which was eventually opened in 1915.

In 1914, numerous special trains were used to carry German troops to their deployment area in the Eifel during the First World War.

In 1945, in the last days of World War II many bridges over the Sieg were destroyed by the retreating German Army.

[2] In the summer and autumn of 2003 the Connex company operated for a short time a long-distance InterConnex service on the Sieg line from Cologne via Siegen, Marburg, Kassel, Berlin to Rostock.

In the spring of 2004, the whole Cologne–Au section was equipped with new high platforms for the extension of S-Bahn line 12 from Cologne to Düren with class 423 rail cars.

Construction of a new station for S-Bahn line 12 at Hennef im Siegbogen began in January 2011 and it opened on 11 December 2011.