The backwards part didn't make me sick (and I'm pretty prone to motion sickness), but we were in the very back car of the train, and I really felt like I was falling forward almost out of my seat. I'm guessing that experience isn't as strong the further forward in the train you go because you don't go as far up the incline backwards.
But my sister, on the other hand, had taken 2 Meclizine (prescription motion sickness pills), and she was very very close to throwing up after the ride. I'm not sure if it was the backwards or just the entire ride, but she was very sick to her stomach.
Just thought you might want another opinion.