In France there are 2 types of railroad systems, public and private. This is where you will see the greatest difference. You can check out trip costs at http://www.europrail.net/index.html This is a good site to make 1st to 2nd class comparisons. We always buy a 1st class pass when we travel and then just pay for seat reservation whenever possible. Sometime you can not reserve a seat, other times you must, it depends on the train. I find 1st class more confortable and less crowded. Train travel can be difficult if not properly.

If you have some specific questions feel free to e-mail me. Have a good trip.

Greg