Originally Posted by
tom tulpe
Do seat reservations online with the respective train operator.
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.
Originally Posted by
tom tulpe
And Euskirchen is a small town. I'd be surprised if there is a ticket office there that's open outside of peak hours, but I'm sure Google will be able to tell you.
DB Euskirchen station