Brussels to Rothenberg via Rhine cruise in 1 day?

I don't think it can be done. I see the earliest arrival in Rothenburg at 9 pm, and the station is outside the walls, so there is a bit of walking still to be done. You might skip the Rhine cruise and be satisfied with the view from the train, which is excellent. Schedules are at: http://reiseauskunft.bahn.de/bin/query.exe/d
Enter St Goar as a stopover to get routed on the local train along the Rhine.

No. It's too long a distance to spend time on a river cruise in between.

There are so many lovely medieval towns in Germany. Could you skip Bruges and substitute, say, Cologne for Brussels this trip and train down to Bingen in order to boat DOWNSTREAM to Bacharach or Boppard? It's crazy to train so far for the short boatride you list. Also, I didn't think the Loreley was the high point of the boat view. Aachen is another idea on the way, but Cologne has even better museums than Brussels.

Are you going to Rothenburg ob der Tauber just for the Nighwatchman's Tour? I am not experienced in travel in Germany, but did the Nightwatchman's tour as part of a Rick Steves tour. While it was fun and interesting it is only about an hour to an hour and a half long and really not worth traveling across parts of 2 countries to hear. I was there the last week of August in 2014 and the crowd for our walk was quite large, probably nearly 100 people for the English language tour. I didn't see how big the crowd was for the German tour which I think starts at 9:30. While I did like the cute factor of Rothenburg (plus the absolutely gorgeous Holy Blood Altarpiece in St James's Church), I think I liked the Bacharach/St Goar area better.

That is just a "my 2 cent's worth" opinion from someone who does not know Germany very well!

Without looking at the exact KD schedule, I can tell you that you'll probably add about two hours to the trip by taking the boat from St. Goar to Bacharach. The current is very strong and the boat goes slow. It takes 1hr 10 min from St. Goar to Bacharach. I put in a schedule having you leave Brussels South on the 6:25 AM ICE, changing trains at Köln and Koblenz and getting to St. Goar at 11:19. You'd have a little over 2 hrs (11:19 to 13:36) to get up to Bacharach, for the next regional train to Frankfurt, then trains to Würzburg, Steinach, and Rothenburg, arriving in Rothenburg at 1 minute before 7 PM. The nightwatchman tour starts at 8 PM. If you knew exactly where you were going and had accommodations near the Market Square, I think you could make it.

Unfortunately (I looked at the KD schedule), the first boat out of St. Goar after 11:19 is at 12:20 and it gets to Bacharach at 13:30. That's not enough time to get to the station in Bacharach at 13:36. So, no. Close, but no cigar.

Spend a night on the Rhein - Boppard or St. Goar. Take a peek into the Köln Cathedral on the way. Take the train or boat to Braubach and see the Marksburg, the only castle on the Rhein that has never been destroyed. Then go the next day to Rothenburg and have enough time to see some of the town before the Night Watchman Tour.

You all are confirming what I was figuring after reviewing the train schedules.

Sam – For what it’s worth, I have sort of fallen in love with the idea of a Rhine cruise over a train. Our entire 2 weeks will be spent on train and this would be a nice, albeit short, break from the rails.

Tim – Bruges is non-negotiable. If anything were sacrificed, it would be the overnight in Brussels. If we make an overnight stop sooner than Rothenberg, that would give us more time to stop in Cologne. Something I haven’t given enough thought to. I had just considered a quick stop just to see the Cathedral and hop back on the train.

Pam – no; colleagues and friends had suggested Rothenberg as a great medieval stop en route to Munich where we will spend the last 5 nights of our trip. Are you suggesting that we might enjoy an overnight in the St. Goar/Bacharach area just as well? The next morning we could continue on to Munich (via Rothenberg?).

Lee – your specifics are always so helpful! If we decide to stop for the night around Bacharach area, your approach to getting here from Brussels helps me a lot. Logistics of train travel still confuse me and I’m never sure which trips to schedule in advance and for which trips we should just buy tickets at the train station.

"Logistics of train travel still confuse me"

Why? It's actually easier than planes, and we use them comfortably over here. They're bigger, so more seats - less need for reservations. They run more often; often once an hour. Their network is much more extensive - they go almost everywhere. Just takes a little getting used to.

Stephanie, I am still new at this and yes, it confuses me too as to what train tickets to buy ahead for a better deal and which ones I can buy at the time where the price doesn't increase the closer you get to your travel date. For me, it also feels like complicates things when you are going from one country to another.

My inexpert opinion is that you should stop somewhere along the line to enjoy what you are doing instead of making a huge push to stay overnight in Rothenburg.