That's a serious overgeneralization. It's true for some people, but trains mostly don't bump and swerve enough for that to be a significant problem. Finnish trains have lots of seats facing backwards and while they're not anywhere as fast as something like a TGV, they're still often going 200+ km/h. People seem to be just fine. I just spent 1 hour 40 minutes yesterday sitting backwards, mostly reading a book, with no ill effects.