I'm sure it'll be at least 10x the cost of flights. Train travel is just too expensive.
Trains compete with cars, not with airplanes.
I could go on.
Show me where a car is competitive for any of those routes. In addition, not even plane is competitive if you consider the airport to city transfer and back.
It also saves you, at least in the case of Paris, Europes most ghastly airport experience, which is CDG.