Leaving Heidelberg around 8:46am getting to St. Goar about 11:31 per d-bahn. 1) What is the best side of the train to sit on and should we make reservations? It's slightly under 3 hours, and we'll be doing this the beginning of June. We have rail passes. 2We are thinking of getting off the train at St. Goar and touring the town and castle. Not sure if we have enough time to tour the castle with out timeline or just see it. We have backpacks, R/S size that we read we can drop off at the St. Goar TI. Not sure if that is still true. K-D boat looks like it leaves at 12:55 and not again until 15:55, if I'm reading the schedule correctly. We were thinking we'd get the 12:55 boat. 3) Getting on the K-D boat at St. Goar and going about 1 to 1 1/2 hours to look at castles on the Rhine. Where would be a good place to get off and easily get back on the train to Koblenz? Anybody done this and can share their experience/advice as to how to best do this.
That route from Heidelberg to St Goar (8:46-11:31) uses an IC from Heidelberg to Darmstadt (9:22), and Regional Bahns from Darmstadt to Mainz and Mainz to St Goar. There are no reservations on regional trains. If you want to reserve seats on the IC, it will cost you €4,50 per seat. You'll be on the train for only 36 min. Any place the boat stops will have a train station, so you can easily get to Koblenz. I would stop in Braubach and see the Marksburg. There are both trains and buses from there to Koblenz. It looks like the 12:55 boat only goes as far down (NW) the river as Boppard, at 13:50. You could go there, take the little ferry across the river. A bus stops at the ferry dock at 14:37 and takes you to Braubach at 14:49. Of, if you know where the Bahnhof is in Boppard and really hustle, you could take the 14:08 train to Koblenz and get to Braubach at 15:06. If you wait for the 15:55 boat, it gets to Braubach at 17:20 and the Marksburg tours only go to 17:00. I had email communication with the St Goar TI office a few years ago and they said that you could leave luggage there, but they close for lunch from 12:30 to 13:30. BTW, the train from Mainz to St Goar goes along the left bank of the river, so you will probably get you best view from the right side of the train, although, at Bacharach and Oberwesel, you'll probably want to see the town, which you can do from the left side.
That connection from Heidelberg takes way too long. Why not just catch the 7:55 or 8:25 departures instead and arrive at 9:31 or 10:37, respectively? Even the latter train would give you time to check out the castle and get back for the 11:55 boat north, which goes straight to Braubach (1:20) and you'd catch the 2:00 tour. (BTW, the TI in Braubach will also hold luggage, they've told me.) After that, a 15:53 train takes you to Koblenz - or to Boppard (arrive 16:43) via Koblenz for a look around there. It's a nice town. The cool chairlift ride runs until 18:00 or 18:30, and there are some nice outdoor cafes on the town square. No lockers there either, however. Are you doing something special in Koblenz? I'd rather stay in Boppard any day. Koblenz is mostly just another big city, not a bad one, but nothing terribly special. And the ride into Koblenz the following day or whenever takes only 14 minutes.