Track movement under freight train

[Charlie B]

Actually we spend most of our time on track work to keep it from moving. Unlike structures made of different materials that have to give, the track is supported to keep it from giving as much as possible. That's why you see tampers on the big roads almost constantly. Even at that it is not unusual for track to pump an inch or so, and as long as both rails pump evenly it isn't a problem.
Jointed rail will pump at the joints, which in most cases are staggered, and this can cause cars to rock sideways until they rock right off the track.
Charlie