The difference is that airplanes have three degrees-of-freedom: pitch, roll, and yaw. During turbulence, they can combine erratically which causes the discomfort. On a train, the "up and down" and "side to side" motions are very limited, but more frequent and/or constant. So it depends on what bothers you most. Airplane turbulence can be unpredictable, and when rough, you are saying "when will this end?" and "will it get worse?" That's not the case on a train.