I love the idea. I don't think that in these cases flying has any real advantage, by the time you get to the airport, check-in, etc. it is hardly faster if it is at all, and trains are just more comfortable.
My biggest worry is that the lack of competition means that the train companies have the ability to do what they want with the services and that charge whatever they want for it. I'm hoping this won't be the case and as far as I know the rail system in France is state owned, so the government has a higher level of control over it and should stop that happening.