Originally Posted by
railways
It's not just a question of making seat reservations, though. Many international (and domestic, in some countries) trains require a combined pass supplement and reservation which can only be booked through certain sales channels. The Seat 61 guide explains it, warts and all.
DB Euskirchen station
Having been to Euskirchen (don't ask), it does not look like a place where DB staff are used to doing anything more complicated then selling a SuperSparpreis return to Hamburg. Cologne Hbf would be a better bet for that (but may have longish waiting times for counter service).