Originally Posted by
hwmorth
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.
You just got lucky, maybe because you proactively approached the conductor. Sometimes on the regionals you won't be able to find a conductor.
Rules (if you read German, if not you can use the flags at the top to show a decent Google translate version):
http://www.bus-und-bahn.de/bestimmungen.html