So I need to get from Dusseldorf airport to Lille-Europe at the end of May ... I haven't used the rail system in Germany for almost 15 years and confess that I feel out of my comfort zone here in understanding the rules and regulations ... I understand that I have to take the exact ICE/TGV train I have ticket for, but that for regional train connections I have some flexibility. What I am not clear on is whether this also applies if the first train is a regional train.

What I would like to do is DUS - Koeln (around 10:00), spend a couple of hours there, then pick up an ICE train from Koeln at 13:43. So basically I would like to book:

dep. 12:50 DUS->Koln[RE 28517] arr. 13.29,
dep. 13:43 Koln->Bruxelles-Midi[ICE210] arr. 15:35,
dep. 16:17 Bruxelles-Midi->Lille-Europe[TGV9836] arr. 16:53,

but then instead take the first (regional) train available to Koeln after arriving in DUS at 09:30 (rather than waiting for the 12:50 train).

Does anyone know if that is that allowed?

Yeah, it's allowed.

If you feel more comfortable, you can also force the earlier train by putting in a connection in Cologne in the search, adding a stopover of 2h (or whatever length is appropriate).

Should be no problem at all.

Thanks all for your replies - very helpful

This was exactly my concern ! But the way you phrased it was much clearer (i.e. the key question is when does the ticket validity start?)

Thanks. I did find the "add stop" option on the DB website, and must say that I was very impressed with this function (more so that it even allows you to tweak the result to get earlier or later connections). Unfortunately though for everything I tried (even without adding a stop) the DB site won't give a price for any of the routings it suggests (I assume because of the combination of RE, ICE and TGV). It does give a price for routings starting in Dusseldorf Hbf - but that would just add a needless stop to my journey and raises the price as the train to Koeln is also an ICE train.

Good to read - that's what I'll do then. The uber-safe option I guess is to just buy the ticket from Koeln to Lille Europe and then buy a ticket DUS to Koeln on the day. As the price though for the connections I am looking at for DUS->Lille-Europe is exactly the same as Koeln->Lille Europe I'd rather save the unnecessary cost of DUS-Koeln and put it towards my lunch there

Here's what DB says about train-specific travel on saver and super-saver fares:
You may only travel on the long-distance trains (ICE, IC/EC) and on the days indicated on the ticket.
When starting or completing your journey on regional or local trains (RE, RB, IRE, S-Bahn), you are not restricted to specific trains or times. Your journey must be completed by 10 am the following day.

If you start at DUSSELDORF AIRPORT then you still need a separate ticket for the sky train from the terminal to Flughafen station

Is the ICE cheaper from Aachen? You could buy some sort of NRW ticket that will cover all transport from the airport to Cologne to Aachen.

Taking these two things together, another option worth mentioning is purchasing a Sparpreis ticket.

It's going to cost like an extra 3 euros but that way, you're covered for the sky train. You may then start your trip from Duesseldorf Hbf as well (as the Sparpreis includes the +City option which is good for travel between the DUS airport station and Duesseldorf Hbf).

Thanks - routing from Aachen to Liile does bring the cost down by a few Euros - but I think I'll stick with going via Koeln, as there are more trains it seems running to Koeln, and it will be good to see the Domkirche again.

I am not sure I follow your exact point here do you mean that a Super Sparpreis ticket doesn't cover the Skytrain and/or routing via Dus Hbf, but that the Sparpreis does? Or are you referring to a different kind of ticket? Sorry if that is a dumb question!

Out of interest, what is the fee for using the sky train? All the Google searches I run for this come up with "buy XXX ticket so that it is included in the price" and don't mention a cost for the individual ticket.

It's �1.70 to the Flughafen Bahnhof, but if you are going straight to Cologne anyway then that would be �11.90 regardless of where you start in Duesseldorf (as well as many other places in a wide radius)

When bying a Sparpreis ticket it will include the +City option: this gives you one journey on public transport included in your departure and arrival cities. This would include the Skytrain and you would not need a separate ticket, costing 2.70 EUR (it is not a special ticket, you basically need a simple ticket on VRR or VRS, the local public transport Verkehr).

Yes. The regular Sparpreis includes something called +City which the "Super Sparpreis" does not. With +City, you may use all public transport in close proximity to your departure (or destination) station. If you chose "Duesseldorf Hbf" as your departure station, the area in which you may use all public transport for free includes DUS Airport. Therefore, both the ride on the Sky Train as well as the ride on a regional train between DUS Airport station and Duesseldorf Hbf would be free-of-charge.

Thanks - unfortunately for the ICE/TGV trains I need to take the DB booking system is only showing pricing options for either non-flex (Super Sparpreis) or fully flex, with "semi-flex" (Sparpreis) greyed-out as unavailable. Good to know for future travel though.

So in the end I ended up booking exactly what I said I wouldn't book in post #3 - namely a ticket from Koeln to Lille-Europe, with the plan to buy a ticket for Duesseldorf airport to Koeln separately on the day using the DB app. It turned out that by the time I booked this was only a few Euros extra than a "Super Sparpreis" trip starting from Duesseldorf (requiring, in theory at least the extra 2.70 for a Sky train ticket).

Thanks to this thread and the Google searches it has inspired, I now understand what a "VRS price level 5� ticket is, and why you need to buy a VRS ticket even though the journey starts inside the VRR region� I have also learned that GoogleMaps for timetables can require care - you need to actually type "Duesseldorf Airport Station" for it to pick up any trains leaving from the non-terminal station (otherwise it only shows routes that connect through Duesseldorf city, and doesn't list any routes using the main airport station).

Many thanks again to all above for the help and advice - I shall raise a toast to you all as I enjoy a beer + wurst lunch in Koeln before my train to France!

Oh my - I just downloaded the menu and it looks fantastic. I'll definitely be sampling the "Koelsche Kaviar"!

I like very much also that the menu points out "Our fried potatoes are served with bacon and onions" - that's certainly a mantra that I could live by Many thanks for recommendation.