Are you allowed to carry on your luggage on these boats ?

I'm not sure I understand the question? Do you mean can you personally carry the luggage to your cabin - verus having it delivered by staff? Is there something especially valuable in your luggage you don't want out of your sight? Or do you have a tone of luggage and are asking about having it stored?

Yes.

A few years ago my mom and I toured the Rhine on K-D. We boarded at Koblenz and got off at Bingen, with our luggage (carryon-sized suitcases).

Most of the K-D cruises are one day. If you are using them as transportation from one town to another. You can take luggage with you,but there is no service to assist you with it and it could be cumbersome. At many of the stops, the ship is at a dock which may be quite a distance from the nearest taxi, bus route etc. if you want to go on to a hotel.

Sorry, my mistake - I did not mean the cruises, I meant the passenger boats.

k-d.com should answer your question

As for passenger boats you can easily take anything on - i conducted bike tours and we put 50 bicycles and bags on one boat many times - lots of room for luggage - no problem

when i said k-d.com ahead i misread and thought you meant cruises.

Yes. We once traveled from München to Köln by rail and K-D steamer with our luggage in tow. The crew showed us where to stash it out of the way on the lower deck.

Yes Reisie dude - you indeed belong on the lower deck - folks like me on the upper deck

but seats in the front of the boat on the upper deck are often taken up before the boat leaves the first port so boarding early can pay off

though after an hour or so most have fled down to the bar it seems.

I don't see Reisie dude on this thread, so I guess you mean me.

The last time Mme. and I went down the Rhein was in March, 2007. Since the outside air temperature was hovering around freezing, the upper deck was completely devoid of life.

When the boys were with us the year before, we stashed our gear below and sat upstairs on the afterdeck.

Well Robes dude - i did mix you up with the low life Reisie - so i retract my comment about you - you are definitely not a low class dude, IMO

unlike that stella in the cella dweller

They say rides on the k-d boats are free if you have a rail pass. If I had a 5 days pass and were to use the pass between Koblenz and Cochem would it count for 1 day ?

Yes, it will use up 1 day of your rail pass.

But that fact doesn't preclude the possibility of making the river ride one leg of a longer trip. We once went from München to Köln by train and steamer in a day. We got an early train to arrive at Rüdesheim in time to cruise as far as St. Goar, where we rejoined the DB.

It is not just railpasses valid in Germany that are valid for transportaion on k-d boats but any rail ticket written along either side of the Rhine gorge can be used on these boats - rail tickets entitle the holder to ride on the boat for the same segment as they would on the train. At least this is the way it always was and i assume still in. (But Lander tickets i believe are not valid on the boats.)