Train Transportation along Rhine River

K-D runs day cruise ships on the Rhein from Mainz to Cologne. Car ferry? I don't think so. There is one continuous boat trip per day from Mainz to Cologne. Leaves Mainz at 8:45, gets to Köln (Cologne) at 20:00 (8 PM). (Boats are really slow.) Trains in Germany go almost everywhere. You can take a train from Mainz to the Frankfurt airport and go by fast ICE from there to Cologne, or you can take an IC or regional train from Mainz along the Rhein to Cologne via Koblenz. Slower but much more scenic.