Rhein Valley towns

St. Goar to Bacharach? That's not a very long trip, why even bother by boat? You mentioned Boppard... there's a chair lift that takes you up to the top of the gorge, and from here you can take in the postcard view where the Rhine makes a 180 degree turn. A little further up the road, you can also climb up to a wine terrace for a similar view, but not as high. So far, I've found Oberwesel to be the most interesting town on the middle Rhine. Bacharach is the most touristy. The only castle I've explored on foot in this area is Rheinfels. I found it a little more interesting than the average castle ruin, mainly because it's so much larger. I haven't visited it yet, but Marksburg near Braubach looks like the most interesting from the outside... but it's on the opposite bank of the river from the one on which you're traveling.

We liked St Goar and St Goarshausen (right across the river by ferry). Also I would suggest Rudesheim for a visit- it is very touristy- but a very pretty little town with lots of wine gardens. you get a great view of the Rhine by going up the cable car to the Germania monument. the cable car ride is also very enjoyable. A lot of people on this site will tell you not to go there-. But it is very popular for a good reason. (dont stay there- its too busy). We found some nice family run hotels in St Goarshausen- took the ferry to St Goar for restaurants. Also be sure to do the boat trip past the lorelei. you do see the Rhine from the train. We took the train a couple of times , short journeys and a long one from Bonn to Frankfurt

The boat is a lot faster down the river (NW, Bacharach to St. Goar, 40 min) than other way, (St. Goar to Bacharach, 70 min). I found 70 min to be more than long enough. You get to see the Lorelei from mid-river rather than from the river bank with the train, and the toll castle, Pfalzgrafenstein, up close. Plan to spend a few hours in Rheinfels before going on to Bacharach. Boppard has a few points of interest (Roman walls) if you have the time. I spent my nights there in 2004, not to see it, but because I found a Privatzimmer for €25/nt and it was centrally located for what I wanted to see. If the Eurail pass includes Germany, the trains along the Rhein are included, as is the K-D boat. Tom, if you are in St. Goar, there is a ferry ( Fähre) across to St. Goarshausen. You walk up the hill to the Bahnhof and take the train to Braubach. You can take a different ferry across from Boppard, but the ferry dock on the opposite bank is in the middle of nowhere, a 1 km walk to the station in Filsen. Or, you can go to Koblenz, where there are bridges and take the train or bus to Braubach.

Thank you all for your answers!! They helped me form an idea of a plan... I'm thinking something like this: leave early from Köln to Braubach, and take a look around (at least see the Marksburg from the outside) then take a train to St Goarhausen, take the ferry to St Goar and tour the Rheinfels. After that, maybe have lunch in town, take de K-D cruise to Bacharach and spend the rest of the day there. I'd love to visit Oberwessel too, but I think I'm a little too short on time...
What do you think about this plan?? Any other suggestions??

I think you really ought to tour the Marksburg if you are going to be there. After that, why not just be flexible about what you are seeing and doing. The train follows right along the river, you can get off and on where you wish. After all, you are there to enjoy yourself, not follow a time schedule. It is daylight until quite late in July, so you have time to explore any of the small towns you wish.

Natalia: your new plan looks mostly good. Do take a tour of Marksburg! Your question #3: YES. You CAN see the same scenery from the train that you can see from the boat. Trains run on both sides. If you take the boat only to Bacharach, you are missing a lot of the river scenery between Bacharach and Bingen. I suggest that you save some time and skip the cruise. After Marksburg, take the train south past St. Goarshausen - all the way south to Rüdesheim (1 hour.) You'll see a lot of castles on the way. Then take the ferry (not the cruise boat) for 1.80 Euros from Rüdesheim across the River to Bingen. The ferry runs every 20 minutes. In Bingen you can get on a train to head north to St. Goar or Bacharach. You will be able to see all the castles again from a different perspective. What about your luggage/backpack? In Braubach, there are no lockers, but the tourist information office will watch it for you. However, you can also put it in a locker in Koblenz if you plan to return there, or you can replan your day and go from Cologne straight to Bacharach in the morning and leave it at your hotel.

Thanks for the suggestions! I like the idea of leaving my luggage; the problem is that check-in time is after 14.30 h so I'll have to ask if they'll keep my lugagge until then.
Would you say I visit Marksburg instead of Rheinfels or both??

In Germany I often leave my bag with my hotel or B&B host in the morning. I did this again a few weeks ago in Boppard, Germany, when I showed up at 8:30 a.m. No innkeeper has ever refused to hold my bag. I would visit Marksburg first and Rheinfels if you have time. If you walk up to Rheinfels after it's closed, that's not such a bad thing. The view from up there is spectacular.