Not much to add to what Juha wrote, except:

* I'm jealous
* +1 on www.db.de. I use it for most of my international train travel, even though I live in Denmark.
* If you're just hopping a train for the day, I assume you're very flexible. In that case, the regional trains in France and Germany should work just fine. I've done tours in both where I just ride for a while and then catch whatever train comes by, if I have somewhere to be (often with somewhat comical results.)
* Be sure to take your shirt off and run like a madman along side the TdF riders, so we can laugh and say "Hey! We know that guy!" Writing www.bikeforums.net on your chest will help.