As stated, it's not particularly jerky, but there is a little bit of whipping action that can happen. I'm very tall, so my center of mass is quite high with my shoulders, neck and head way above the back of the seat. I do experience a fair amount of thrashing about if I don't slump down significantly (not an easy thing to do). But that's just physics in action and not nearly as noticeable if you're under 6'. If you are concerned about neck issues and "jerkiness" ask to sit in the first car of the train. There is less movement there. The last car in the train experiences additional motion, as excess movement (energy) from the front cars is translated back into the rear cars. Physics again.
Alternately, if you want a bit wilder ride, sit in the last row of the last car.
Steve