If we're talking about the GERMAN part of the Rhine, there are 3 main possibilities (within practicality):
1. In the Interwar period, the Russians beat the crap out of Poland and then take Germany (unlikely due to lack of power projection and the rest of the world will likely jump in to save Germany)
2. D-Day never happens, The Russians have to push to the Rhine, and the Allies jump in only to get France. The Russians occupy the entirety of Germany, albeit at the cost of many lives (they are unlikely to go through with attacking Japan, because they're already severely weakened.
3. The Communists reach win the war of the streets in Germany. Germany and Russia occupy Poland and the French invade the Rhineland. The Russians are asked to intervene on the behalf of the Germans. The Russians fight off the French (somehow?).