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Line S12 is a S-Bahn line of the Rhine-Ruhr network in Germany. It is operated by DB Regio using class 423 electric multiple units. The S12 runs from Horrem via Cologne to Au (Sieg). It shares this route for most of its run with the line S 13. With over 105 kilometres (65 mi), it is the second longest line in the Rhine-Ruhr network, after S9.
Services operate every 20 minutes on working days and every 30 minutes on Saturdays, Sundays and public holidays as far as Hennef (Sieg). It operates hourly between Hennef and Au. Except during the peak, services do not operate west of Köln-Ehrenfeld towards Horrem.
The route runs on the Cologne–Aachen line (opened by the Rhenish Railway Company between Cologne and Horrem in three stages between 1839 and 1841), the East Rhine Railway (opened to Troisdorf by the Rhenish Railway in 1870, with an extension to Cologne opened in 1874) and the Sieg Railway (opened by the Cologne-Minden Railway Company between 1859 and 1862). The eastern section of the S 12 service from Au (Sieg) to Köln-Nippes was opened on 2 June 1991. It was extended to Düren on 13 June 2004, dropping Köln-Nippes. It was cut back to Horrem in December 2015.