I believe the best way to travel any distance in France is by rail. The TGV trains are reliable, comfortable and very reasonably priced. From Paris to the South is quicker by rail than by air.
There is only one rail company SNCF – their website can be confusing, but booking and printing tickets online is not difficult. If you click on the English flag on their site it takes you to the European TGV page which is different from the main SNCF service.
We have also found that their online service is limited if you are planning to explore France by rail, for example, on some routes you an get to a place, but according to the computer there is no return link.
Eurail enables you to buy tickets before you arrive in France, but you will pay more;