I want to experience traveling up the Rhine River in Germany but do not want to do a reverse trip in order to get my luggage. Can I take my luggage aboard The Koln-Dusseldorfer (K-D) Line?
According to their FAQ (see "Bicycles & Luggage"):
There is no luggage office on board or at the wharf, therefore large baggage cannot be transported (max. hand baggage size). The passenger is responsible for transporting their luggage and other items brought on board to and from the ship. The carrier accepts no liability for luggage and other items brought on board.
However, the german version is muss less specific, saying:
Eine separate Gepäckaufbewahrung ist weder an Bord noch an der Anlegestelle vorhanden, daher sollte kein großes Gepäck (bis max. Handgepäckgröße) mitgeführt werden. Für die Beförderung des Gepäcks und der an Bord gebrachten Gegenstände zum/vom Schiff hat der Fahrgast selbst zu sorgen. Eine Haftung für Gepäck oder an Bord gebrachte Gegenstände wird ausgeschlossen.
with the relevant part saying that no large baggage should be brought aboard. So maybe you can take it with you, but I wouldn't bet on it.
If you want to be sure, you should write to them at firstname.lastname@example.org.
Note however, that if they don't allow it, a train trip back will be much shorter than going by boat.