The Cologne–Aachen railway also became increasingly strategic for military traffic to Belgium and for connecting the strategically important Vennbahn. Numerous improvements were carried out from 1912 to 1914 to improve the performance of the line, including the upgrade of tracks and the establishment of an overtaking loop in Derichsweiler. Quadruplication between Langerwehe and Nothberg was planned in 1914, but this was prevented by the outbreak of the First World War. During the war, the railway was used to transport troops and supplies to the western front. After the German declaration of war on France on 3 August 1914, the line was closed to civilian traffic and only used for the transport of troops. Regular traffic resumed in mid-September. The marshalling yard in Düren was extended for military reasons as late as 1917.