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.

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

Of course I proactively told him I need a ticket when he came around. As I said above, it�s important to inform him rightaway that you need a ticket.
If there�s noone around, that�s different, of course, but then that would mean no checks either.

Thanks for the link , I was not aware of the website. If you look at section 7.1, paragraph 2:

(2) Beim Einsteigen muss der Fahrgast einen f�r die gesamte Fahrt g�ltigen Fahrausweis haben. Falls nicht, muss er diesen unverz�glich und unaufgefordert l�sen. Ein Fahrausweiskauf in den Z�gen der EVU bzw. Stadt- und Stra�enbahnen ist dabei nur ausnahmsweise dort m�glich, wo mobile Fahrausweisautomaten eingesetzt werden oder ein Fahrausweisverkauf beim Fahrer von Stadt- und Stra�enbahnen stattfindet; ansonsten ist er ausgeschlossen

RE trains are staffed with DB personnel so they have ticket selling capability.

But in any case, to get back to the OPs question, I do not mean to promote buying on board as the best way - it is merely a decent backup in case time is scarce. Usually, I also prefer the ticket machines.

I do not know whether your reading of German in Switzerland is different from mine in the heart of the relevant state, but the language is pretty clear and it is certainly not what you say:

You need to have valid ticket when boarding regional train in NRW unless:

- there is a vending machine on the train
- the driver of a tram or expedited tram (= Stadtbahn) sells tickets

DB staff is neither "a vending machine on the train" nor "the driver" of the unit, so you need a ticket. Full stop. The conductors on the RE1 have the opportunity to sell tickets a to deal (a) with dumb but bona fide tourists or deal (b) with the situation that there was no vending or a defect vending machine at an intermediate station.

Nice to see this heating up a bit

I was in fact also looking at the passage

"Falls nicht, muss er diesen unverz�glich und unaufgefordert l�sen."

However, in my quest to be right I came across http://www.db-fahrpreisnacherhebung....eit/index.html where it says

"Nachl�sung im Zug
Grunds�tzlich muss der Reisende bei Antritt der Reise mit einer Fahrkarte versehen sein. Eine Nachl�sung im Zug ist nur im Fernverkehr (ICE/IC/EC) m�glich. In Verkehrsverb�nden ist das Nachl�sen im Zug generell ausgeschlossen."

Since this is a DB site I assume it has some sort of final say over the DB procedures (the NRW page being made to cover a wide range of applications, including RUs others than DB, is of course worded in a way that has the common to all operators, but thus, least, rights). This, I assume, puts an end to this discussion.

So, if the OP is still reading: Buy-onboard normally only works on IC/EC/ICE trains. Sorry for the confusion - I was working with my experiences. But of course, I will now go ahead and recommend those IC/EC/ICE trains for their onboard bar with beer on tap

No heat, plain legal language. The web page is crystal clear:

(a) You need to have a ticket for every public transport vehicle....
(b) If you do not have one, you need to buy one onboard....
(c) You can't buy one onboard of trains and trams....
(d) unless the driver sells tickets or there is a vending machine.....

Another option to keep in mind during planning is staying the 2nd night in Duisberg and catching a train or taxi down to DUS in the morning.

Why would you want to do this? It might be less expensive than staying in Dusselsdorf or Cologne, depending on if there are any special events going on in either of those towns.

Depsite the fact, why would someone stay there, it's Duisburg.

If etch5895 goes to "Dusselsdorf", he could give "Duisberg" a try, too. Nothing wrong with that city, I had to spend close to two years with the local court there (and survived) and the remark about (a) hotel rates and (b) the proximity to "Dusselsdorf" is certainly correct.

You've got me. I've got a pretty bad head cold and I wasn't paying attention to my spelling or my umlauts.

Well, if he is only in town for a conference or meeting, and doesn't really feel a need to stay in the latest trendy D�sseldorf hotel at 200 euro a night + 50 euro for the meal and drinks, but instead opts to bed down in the Ibis Duisburg at 60 euro a night plus 6 euro for the D�ner and beer, he will probably come out ahead financially. His trip seems too short for much sightseeing.

Agreed, however if so, the Sheraton at the airport or the Holiday Inn in Ratingen will certainly do the trick too. The Sheraton is in the airport and the Holiday Inn offers a free shuttle.

Stayed first night @ Cologne-Mullheim HI express, traveled a bit near the train station when I was done with my meetings.
I really liked the luggage storage @ the train station!

Took the train to DUS Hbf and stayed in the HI close to it, walked to the old city area - man, you DUS guys know how to party - the area was PACKED with people going out, eating, drinking and generally having some good time.

Ended up taking a taxi in the morning to DUS, spent some time in the Biz lounge in which I declined the many alcoholic temptations so early in the morning...

The fare (hbf to hbf) appears to be 9,80 for the RE, 11,00 for the IC/EC, and 16,50 for the ICE. If buying a single ticket and the next train was an IC or EC, I'd take that rather than wait around for the next RE.

Sounds good, you got the best out of it.

Thanks for exactly reporting back, I wish so many more would do that! ^^