Line S 13 was an S-Bahn line of the S-Bahn Köln network. It was operated by DB Regio with class 423 electric multiple units.

Köln Hauptbahnhof is the central railway station of Cologne, Germany. The station is an important local, national and international transport hub, with many ICE, Eurostar and Intercity trains calling there, as well as regional Regional-Express, RegionalBahn and local S-Bahn trains. EuroNight and Nightjet night services also call at the station. It has frequent connections to Frankfurt by way of the Cologne–Frankfurt high-speed rail line, which starts in southern Cologne. On an average day, about 280,000 travellers frequent the station, making it the fifth busiest station in Germany.