It depends on the routing especially as you get to traveling over borders in Europe. It does seem to be steadily improving though. Just a few years ago, I was going from Dusseldorf to Paris and I was rather surprised that train really wasn't a very good option.
Today, as I go to whip that out as a counterexample, I see that Thalys would make a whole lot of sense vs. flying for me (though I don't know how door-to-door costs would compare).
The Marseille/Avignon area to Geneva, Frankfurt or pretty much anywhere in Germany is surprisingly frustrating. I can fly from Marseille to Frankfurt in barely an hour -- a train ends up eating most of the day.
The only time I take the train over flying in Europe is typically Avignon-Paris. Everywhere closer and it's faster to drive, anywhere further and it's faster to fly.
Today, as I go to whip that out as a counterexample, I see that Thalys would make a whole lot of sense vs. flying for me (though I don't know how door-to-door costs would compare).