With the dual-class Intercity system of DB introduced in 1979, the line benefitted at the edges of the day: a morning IC service from Cologne to Hamburg already started in Aachen, stopping in Düren, and connected in Cologne to become part of the IC regular interval pattern. It was given the name "Karolinger" (after Charlemagne) and returned from Hamburg to Aachen in the evening and remained a constant in the timetable for over two decades. A real cyclical service pattern did not develop between Cologne and Aachen until 1984, when the Belgian State Railways introduced its new nationwide timetable concept, which included a service every two hours from Cologne via Aachen and Brussels to Oostende, which was operated with air-conditioned Eurofima coaches. The rest of the services operated, on the other hand, was only switched to regular intervals when the regional rail regular interval timetable was established in 1991.