I should have clarified. The tickets I'm talking about, which you can get from the Köln Hbf DB office, or any DB office, are open ended, either direction, with U-Bahn connections included. As such, they are not tied to a specific date or train, so they want to make sure you time stamp (the best term I could come up with) so that, if the ticket isn't checked, you cannot reuse it. Long winded explanation...
In practical terms, as you board U-Bahn train, there is a time stamp machine in every car, it has a little slot for the ticket (similar to a parking lot time stamp), insert ticket, and it'll print time and date. If you board train at Köln Hbf, any train to DUS, those little machines are at the bottom of the stairs/escalators before going up to the train platforms. In DUS, the machine is at the top of escalators, before descending to platforms. Once you look for them, they'll be more obvious.
I learned this to avoid having to get tickets tied to a specific date or train. Open ended tickets work easily. You just look for the next departure on your DB Bahn app, and board.