1 day castle River trip from Cologne area to Heidelberg?

There are no day cruises that cover that distance. But you can use the train, combined with a cruise and a castle tour, to have an excellent day. See outline below using current rail schedule. KD Cruise Boat schedule from high season 2019 (should be similar in the future.)

• Take the train from Cologne to Braubach. (8:17 - 10:05, for example.) Drop bags in a locker at Koblenz where you change trains prior to Braubach.

• Tour Marksburg Castle - best example of a medieval castle on the Rhine (figure 3+ hours including the walk to the castle and back.) Maybe have lunch in the old town.

• Catch 13:54 train to St. Goar (arrive 14:55) via Koblenz (pick up bags from locker during 24-min. layover in Koblenz.)

• Cruise from St. Goar (15:20) to Bingen (18:00.) This is the most scenic part of the Rhine.

• Bingen > Heidelberg by train after that.

Thank that sounds great!

Ditto Russ.

Some trains from Cologne stop at every town. Don‘t take one of those.

Some trains don‘t go all the way to Koblenz. We got on one that had only some cars continue to Koblenz. We had to get out of our car in Remagen and get into one that continued to Koblenz. Usually this is indicated on the car but I did not see it. Only because I could somewhat understand the German from the poor PA system did we realize what we needed to do.

Are there any private small boat companies for a one way middle Rhine tour in June?

I don't know a specific answer - maybe Russ will - but I will say that in many years of being alongside or on the Rhine in that area I can't directly ever remember seeing "small boats". The cargo ships are very large, the tour boats are multi storey and quite large, and the cross river ferries are big enough to carry a couple of dozen cars and trucks - I would think it unsafe for small boats.

The KD ships go either way. Get on one of those. If they hold 100 px, I would be surprised. Even the river boat cruise ships only take about 100-120px. They are not huge.
You can get on a KD ship in St. Goar or St. Goarshausen and take it to Bingen or Rüdesheim and then continue by train.

There is a small company doing speed boat tours for a high price, but don't think that is what you are looking for.

Different trips have different needs, but the availability of bag storage and food and drink on KD ships can be counted as an advantage. FWIW, I found that the downstream (northbound) boat ride was NOT too fast to enjoy the scenery. Don't overlook the need to carry your bags from the KD dock to the train station, wherever. Sometimes a stairway across the train tracks can be necessary. And the (presumably still in effect) KD discount with a same-day train ticket.

(Cologne is a favorite destination of mine. Do not consider reducing time in Cologne/Duesseldorf to facilitate the river day. The local boat ride in Cologne is not very interesting.)

Is there much to see between Bingen & Mainz on the ferry? We will board the ferry in St Goar.

It is a river tourist boat. A ferry is what takes people and cars from one side of the river to the other. There are several on this stretch of the Rhine because of the lack of bridges.

bobbie, all the significant scenery is between Bingen and St. Goar, known here as the scenic Middle Rhine. You have to look at the boat and rail time schedules to determine your best use of time if your destination is Mainz. You need to consider how little there is to see near Cologne. Koblenz has a little scenery, but the boat is too slow for such a long trip, especially upstream. Even the most local train is multiple times faster.

I agree that the frequent use of the word cruise to describe the KD day-boat is confusing. But it is not a ferry. And in my opinion it is not frequent enough to be treated as a city bus to visit multiple towns and castles in a single day.

https://community.ricksteves.com/travel-forum/germany/best-way-down-rhine-from-dusseldorf-to-frankfort-in-1-or-2-days

https://community.ricksteves.com/travel-forum/germany/mosel-valley-cochem-or-trier-or-west-rhine-valley-mainz-for-3-4-nights

Thank you everyone for your wonderful suggestions. We will take the train from Cologne to St Goar then KD to Bengin then train to Heidelberg. We are getting excited for our June trip.