I suspect it's fairly common to become less carsick with age (it happened to me as well, and it's not like I trained -- I hadn't read in a car for years before trying to do that again and noticed that it bothered me much less). Anyway, in my case the problem is not sickness (I don't get sick at all when on rails), but just that I can't concentrate very well when on a train. So I can read short stories or poetry no problem, but I usually don't even try to read textbooks or papers.