I never really thought about doing it before. the last time we used amtrak we took the auto train so it wasn't an issue. If you do, can you check them in as luggage or use them in the seats?
I just got a reply on a post I made asking this same thing & everyone has said that there are no seatbelts on the train. I would think that you could check them. They don't count things like that & strollers as part of your limit of checked luggage.