Russ: You win. This is of no help to the OP. Now I'm off to finish my Christmas shopping.

Thanks for all the info. By looking at a map I can see that some towns are on one side and some are on the other side of the river. This did indeed help when looking up train schedules. Clearly it's easier to take a ferry over to the "other" side and then resume train on that side. I think I have it figured out.

Russ, thank you so much for the info on the Rheinland-Pfalz Ticket. That certainly will make it less expensive than a car so I think we'll go that way. We do tend to drive a lot on trips and I love the flexibility driving provides but it seems I can get nearly everywhere I want by train and won't have to worry about parking.

It looks like in most cases there's some form of transportation from the riverside towns up to the castles if you don't feel like walking. Does this sound right? I guess the big exception is Burg Eltz which seems a lot harder without a car.

I'm thinking it makes most sense to base in Koblenz, perhaps for four days and do day trips up and down both rivers, then move to Cologne for two nights to do Aachen from there, and be able to get to the Dusseldorf airport for departure. Do this sound reasonable?

The R-P ticket requires that you use the RB and RE trains - not the high speed ones - which is really just fine since you're not traveling long distances in this area. From Koblenz to Trier, your longest run, catch the RE, not the RB, which stops at every town and takes substantially longer. Also - on weekdays, you must use the R-P only after 9 am; travel prior to 9 requires an additional ticket. You can buy the R-P ticket from one of the red/blue DB ticket machines (they have an English interface) and use a credit card.

Your plan sounds reasonable to me; in Koblenz, try to stay somewhere within walking distance of the station to minimize your coming/going time.

Burg Eltz is not a difficult walk but it is time-consuming. Marksburg and Rheinfels have shuttles but can be walked to as well. I've always walked on my visits. Cochem's castle (Reichsburg) isn't hard to walk to either, and it has an excellent falconry show (separate admission.)

If you should decide to drive (and we much prefer the flexibility that driving allows along the Mosel), parking is easy and free in most of the smaller towns. Those few that do have pay parking (if you were stopping for just a short visit, for example) have plenty of it and we didn't find it at all expensive.

burg eltz should not be missed. i am not sure how to get there with no car. a taxi?

i also like having a car to be able to sleep "off" the busy mosel/rhein when i want to .. back towards the lovely farmlands. it is very peaceful.

In September I traveled exclusively by train and spent four nights in Cochem. It's a great base being centrally located between Koblenz and Trier with better train connections than many of the smaller towns.

To get to Burg Eltz I took a local train to Moselkern - a very small town. There is a walking trail to Burg Eltz from Moselkern - it's supposed to take one hour to get to the castle. The station was deserted and I was the only person to get off the train there. The signage was not the best. I walked for 20 minutes before I saw anyone to ask if I was going the right way. It took me just under an hour and a half to get to the castle, much of which was on paths through the woods. If you have mobility problems this is not the way to go.

The castle itself and the setting are worth the trip. There is a parking lot not far away and it would be a much easier trip by car.

It may be worth your while to rent a car for the day.