The Cologne–Overath railway is a single-track, non-electrified railway in the German state of North Rhine-Westphalia. It was opened in 1910 to connect the historic Siegburg–Olpe railway directly to Cologne and required the construction of the Hoffnungsthal tunnel. The section from Hoffnungsthal to Rösrath partly used the track of the Cologne–Lindlar railway, which is now largely disused west of Bergisch Gladbach. The line, like the Siegburg–Olpe line, is also called the Aggertalbahn (Agger Valley Railway), although it leaves the valley of the Agger not far from Overath.