I'd plan to take a morning train from Amsterdam to Koblenz - would arrive at Koblenz around 11:45am April 12 so hope to use the afternoon of the day to visit the best part of the Rhine ie from Boppard to Ruedesheim. I'll hope to use the morning of the next day to visit Koln and be back to Amsterdam around 6pm. Any suggestions on where to stay overnight (Koblenz?) and itinery? The schedule I know is really tight, do you think i can fit Burg Eltz in? many thx!!

Suggest you train to Koblenz, check in (or at least drop your bag) near the station, then hop a train south along the east bank to Braubach for a tour of Marksburg Castle - a good alternative to Burg Eltz in your situation as it's right on your route and is a good bit easier to reach:

www.marksburg.de

Then proceed south to Rüdesheim. You won't have time for much along the way but scenery, but it's very good scenery. It'll likely be dark for your trip back to Koblenz, but if not, perhaps you could ferry across to Bingen, and train back to Koblenz along the west bank for a different view of things.

Its not possible to get Eltz into your itinerary. There are no buses or trains going to Wierschem (which is the place next to the castle) on weekends out of Koblenz.
I would not stay in Koblenz for long or spend a night there. There isnt much to see or do in that town.
Take the next train (leaving Koblenz main station at 11.55 am) to Ruedesheim or stop at a station on the way as St. Goarshausen or Kaub. Time to go to Ruedesheim about 1 hour, the other destinations less. A train next morning from Ruedesheim to Cologne will take around 2 hours with interchange in Koblenz.

I agree with wolfmisc that Koblenz - though nice enough - isn't a destination in itself. But it has plenty of hotels and restaurants near the station and makes for a good place to stay if you just want to get to Cologne quickly the next morning, and if you plan to see Marksburg in Braubach on day 1 - there are no lockers in Braubach to stow your bags while you do the tour.

Many people take the train to Moselkern, which does run on Saturday, and then hike several km through the woods up to Burg Eltz. But, having been to Marksburg, I would agree with the recommedations to go there. It is the only castle on the Rhein never destroyed (Burg Eltz is on the Mosel), and is furnished as it was hundreds of years ago. It is also more accessible. There is a shuttle from the K-D dock in Braubach to the castle.

The VRM website (Verkehrsverbund-Rhein-Mosel->www.vrm-info.de) shows bus service from Koblenz to Weirschem, even on weekends, but it is fairly round about and takes 90-120 minutes. And, it looks from the map like the walk to Burg Eltz from Wierschem might be as far as from Moselkern.

Sorry, I wrote "Weirschem"; it should be "Wierschem". Looking at a topo map, it looks like the walk from Brunnen bus stop in Wierschem is pretty flat. I understand that the hike up from Moselkern is a climb.

The Rhein will be a full day. Focus on the area from Bacharach to St. Goar. You can walk the old towns, ride the cruise boats for the one hour trip between the two towns and explore Rheinfels castle. You might also be able to get Marksburg into your itenerary.

Burg Eltz is on the Mosel. It's really not practical to try to get some Rhein and some Mosel in one day. With two days you could take an early train to Trier, explore the Roman ruins and market area, then head to Moselkern, hike to and tour Burg Eltz.

While Koblenz is a convenient transition between the two. I'd rather spend the night in Bonn, Koln or Burg Stahlek (a castle turned hostel in the area of Rhein mentioned above). Local trains work well in these areas but check the schedule twice for limited service on days like Sunday or weekday evenings. I tried to catch a train that didn't run on a weeknight and ended my day with a long hike back to my pension.