Short answer: I don't know. Motion sickness on a train is rare, but some people may be affected.
Long answer: Anecdotes.
1. When Amtrak was doing the research that led to development of the Acela, some European trains were tested in the US. A friend of mine was on special assignment, involved in passenger service on those trains. She said she noticed that at least one of the European trains had permanently mounted seats, with half the seats facing forward and half backward. When she asked the traveling European representatives about this, they were surprised at the question and said the issue had never been raised before. When I have heard this issue brought up on Amtrak, the affected passengers have usually been (apparently) of Northern European descent. My question is, why are northern Europeans affected here, and not there? Of course, there may be factors that I'm not aware of.
2. Many years ago, I was seating passengers in one of our old Heritage Buffet diners. We were 100% booked, and had to use every seat. A lady came in and said she had to sit facing forward. I directed her to a forward-facing seat. It was dark out, and we were between towns, with no lights, so there was no way to visually see which way we were traveling. "NO, NO," she said. "I need to sit the other way!" and she sat down in a rear-facing seat. I wasn't about to argue, and continued to seat passengers until every seat was occupied. Then we came to a small town where the streetlights were on. I guess she figured it out, but I didn't address the issue further. She chose not to trust me; she chose her own seat; and by then there was no other seat where I could move her. She didn't get sick, and never complained.
I personally believe it's more in the head than in the stomach, but I'm not really qualified to guarantee that I am right. Whatever the source of the problem, it is real in a very small number of cases, and I don't want to cast aspersions on those who are affected.
Tom