From Freiburg im Breisgau via? Cologne ? Trier? Koblenz? to Dusseldorf by train

I would suggest stopping in Boppard, or one of the other Rhine towns. Cologne is a large city, Trier is very much out of your way, and Boppard would have more of a small town feel to it than Koblenz. You could hop on some of the river cruises offered, and visit the many castles in the area like Burg Eltz and Markham.

I spent 4 nights in Boppard in 2022 and really enjoyed my stay there. It's easy to get to by train, the town center is a short way away and there are plenty of restaurants, cafes and shops there. And the town actually sits right up against river, so you can easily walk down to one of the cafes there and have a drink while you're watching the boats go by.

BTW, if you're going to write in German, make sure you capitalize the nouns. Vielen is capitalized because it's the first word in the sentence, but Dank would be capitalized because it's a noun. :-)

Cologne isn't really of interest unless you're into medieval churches, and Trier is very out of your way. Heidelberg would be a convenient stop, or Frankfurt.

thanks Mardee and Phillip. I'm liking that Boppard rec a whole lot, the river, hiking. Heidelberg is good too, although I'm not much into dueling. ken

It's a nice place and as I said, I really enjoyed my stay there. I stayed at the Bellevue Rheinhotel, which sits right on the water, so you can walk out of the hotel and the Rhine is right there. It's also right by the KD dock, which is one of the major companies that offer Rhine cruises.

One other advantage is that if you stay in Boppard, you can ride the train locally for free. You get a card from the hotel that will entitle you to free fares within the VRM transit authority.

When? Things are still open, but once the tourist river boats stop (within the next 2 weeks) all the small towns on the Rhine become ghost towns.

You want to go to either Luxembourg or Mainz. Both of those have the rail and public transport to get you around, the route is fairly direct, and they have enough to do in the city or its immediate environs to keep you entertained.

You asked about Koblenz... Koblenz does not turn into a ghost town in October. It's a large-ish place and a busy one year-round. And unlike Luxembourg, it's in Germany, and it's located where both Rhine and Mosel rivers meet up, and it's on the route to Düsseldorf that you asked about. Outings from Koblenz are pretty simple thanks to its location and train-hub status. Tracks from Koblenz head out in every direction:

https://www.vrminfo.de/fileadmin/data_vrminfo/PDF/RLP-Ticket_streckennetz.pdf

Trier, though some distance from Koblenz, is a feasible day trip from a Koblenz base.

I would give Koblenz primary consideration if you plan to be in the area after October, Boppard before that.

Also, you could arrive in Koblenz (or in Boppard, or in one of the other smaller Rhine towns) as late as the last week in October and still take a river cruise. Bingen-Rüdesheimer's "Loreleyfahrt" boat schedule ends on October 27:

https://www.bingen-ruedesheimer.de/home/

Date of trip?

Cologne has a lot to offer, not just churches. It's a city visit, not a riverside town that's shut down for the shoulder season.

https://community.ricksteves.com/travel-forum/germany/can-i-see-enough-of-cologne

best of advice will be possible when we know when, how many, mobility issues if any, and what would make a town "worthwhile" for you.

Dueling isn't mandatory

Everyone your advice has been great and Veilen Dank again. (That's pretty much the extent of my German and initial caps it is.)
Pulled the trigger on Boppard Oct 16-18. Still have day and a half north out of Freiburg to fill. Might just stay longer in Freiburg (I'm there two and a half days) depending on weather and Black Forest biking or squeeze in Heidelberg. Any more thoughts always appreciated. Ken

Still have day and a half north out of Freiburg to fill.

There are some very good options between there and the Rhine Gorge... depends on your interests, which we don't really know... Have a look at Tripadvisor or other sources for info on Stuttgart (Mercedes and Porsche museums are popular there among other things) Speyer (Technik Museum, cathedral, Jewish history) and Mainz. (Or if you happen to share Rick Steves' affection for spa treatments and casinos, you might take a glance at his paragraphs on Baden-Baden.)

I don't have nearly the expertise or experience of many of the posters on the Forum but I have greatly benefited from their sage advice especially regarding Cologne, Freiburg, and Boppard/Rhine region.
This past July we stayed in Boppard at the Bellevue in an apartment just down the street from the hotel. Like the hotel, the apartments were right on the river. Nice places to eat (lots of Italian places which surprised us). We enjoyed the Roman ruins as well as several visits to Backerei Martin Nickenig where we enjoyed several nice continental breakfasts. Much less expensive than the breakfast buffet at the hotel. In addition there's a beer garden about a 15 minute walk north along the river where we watched the Cologne soccer team play and enjoyed a nice meal.
As for Freiburg back in 2019 we took a tour of the Black Forest by car with Simone Brixel. She runs Black Forest Tours and is recommended by Rick. She tailored a tour just for us - Triberg waterfall, cuckoo clocks, butcher shop... It was very informative and pleasant.

We returned to Freiburg (along with Cologne and Munich) this summer and we took a couple tours of the town with two other guides. One a food tour of the market and another a walking tour of the Old Town. Both were about three hours long and very informative. One recommendation was to visit the cathedral museum where original parts of the cathedral are on display including statues and gargoyles. Very impressive in a unique setting.
We also took a Flix Bus to Strasbourg, France for about 50 euros total for the two of us. Drive was just over an hour and it was a 20 minute walk to the cathedral. Good food, nice shops, canal cruise - picturesque indeed.