Originally Posted by
soitgoes
That's fine for IC, EC, and ICE trains, but regional trains require tickets prior to boarding. On these routes there is no ticket collection, but, rather, spot ticket inspections.
Have to disagree there. I have bought a ticket onboard the RE 1, was traveling from Cologne to DUS. Must have been sometime in January this year.
Not sure about S-Bahn trains though, but I advise against taking those between Cologne and Dusseldorf anyways.
But, as said before, I do agree with the previous posters that buying onboard is more of a last-resort measure.