The Sieg Railway (German: Siegstrecke) is a 100 km long, electrified German main line railway between Cologne-Deutz via Porz, Troisdorf, Siegburg, Hennef, Au (Sieg), Betzdorf to Siegen with a through service to Cologne Hauptbahnhof. Although most of it is two-track, two five-kilometer sections are only single track. Both ends of the line are in the state of North Rhine-Westphalia, but between Au and Niederschelden it runs through Rhineland-Palatinate. It is one of the oldest lines in Germany, opened between 1859 and 1862 by the Cologne-Minden Railway Company.