I'm sure it'll be at least 10x the cost of flights. Train travel is just too expensive.
Flying is just too cheap ;) It's easy to say, but if you look at the environmental impact, it really is too cheap. If airlines would pay the same tax on fuel as cars and were expected to compensate for their pollution, they'd be a lot less attractive.
The other factor is that air travel is consistently profitable in the EU, without subsidies. But the reverse is true of rail.
The simplest solution is just to impose a $100/ton carbon tax, and let the market figure it out. But EU bureaucrats seem intent on socialism and planned economies instead.