What is best route from Venice to see Alps and end up in Rhine River are of Germany?

How many days do you want to take for that journey - all in one day or can you spend a few days on the way?

When you say Rhine River area - do you mean the Mittelrhein where Rick's river castles are, or do you mean the upper Rhine north of Basel and near the Black Forest? Or further north around the Dutch border?

Hi,
By "Rhine River" I'll assume the scenic stretch between Mainz and Koblenz.
First off, how long do you have? If only a day, the only viable option is the Venice-Milan-Basel-Mannheim-Mainz route, with two variants between Milan and Basel (via Simplon tunnel, or via Gotthard tunnel). Both are somewhat scenic in parts, but use long tunnels that bypass the really scenic strecthes.
But if you can break the journey, then Bernina Railway is probably the most scenic route : you go from Venice to Tirano (either via Milan or via Bergamo & Lecco), change to the Swiss trains over Bernina pass to Chur, then take whatever train is fastest to Germany. You'll need to break the journey somewhere, probably in Switzerland