I have been on trains that didn't bother me in the least, and also trains that did. The difference seemed to be the level of air-conditioning! Riding third-class in Asia was totally fine-- open windows, ceiling fans, wooden benches, and sweating our brains out... was a lot of fun. But Amtrak in New England... was exhausting. Funny enough, that third-class car, there did seem to be a lot of local people suffering greatly from motion sickness. You could tell because the local remedy involves sticking scraps of paper to your face-- there must be some oil or unguent involved but I never did discover what it was. Wish I knew.