[gstipi]

Your best case scenario is actually a worst case scenario.

From Düsseldorf to Frankfurt, using regional trains, you have to change once in Koblenz (RE 5 to RE 2). This is a connection which takes 3 hours and 50 minutes and runs every 2 hours (with numerous other options in between).

[LargoLasskhyfv]

Also rather scenic if it takes the 'left side' between Cologne and Koblenz. Escpecially in the upper floor of the 'Dostos' (Double decked coaches). The Rhine, all the old castles, the wine grows on the steep hillsides, maybe some ships...