After a change to one of the dual voltage sets on service S4, we returned to the Abtalbahnhof, and then took the connecting curve onto DB tracks where we passed the voltage change over point, which automatically switches the car’s systems to 15kv a.c.

http://www.lrta.org/mreport14.html

If the changeover point from 15kV to 25kV is somewhere on open track between Aachen and Welkenraedt, multiple-voltage (or diesel) motive power will be needed again for night-trains or seasonal trains. But if it remains at its current situation (Aachen Hauptbahnhof), locos will have to change from 15kV AC to 3kV DC in Aachen and from 3kv DC to 25kV AC a dozen kilometers further on!
http://www.bueker.net/trainspotting/lines_cologne-brussels.p...
May 1966 saw the introduction of electric traction between Germany and Belgium. On the DB line Köln-Aachen 16 2/3rds Hertz, 15kV AC power was used, whilst the voltage on the Belgium section Liège-Aachen was 3000V DC. Aachen HBf was chosen for conversion to a system changeover point, as the tracks and platforms were no longer sufficient for the increasing international traffic.
http://www.bueker.net/trainspotting/stations_aachen.php