When I was in grade school, maybe 3rd or 4th grade, all the grade school kids, probably 120, got on the train at our small town and rode it 10 miles to the next town. Road in box cars with both side doors fully open. Took turns sitting in the doorways with our legs hanging out.
What I remember most is how badly the cars rocked back and forth. Looking forward or rearward it appeared the tops of the cars were moving a couple feet. It was a struggle to stand up without taking a step to catch yourself.
So, if the rocking action got in unison..... Maybe this is what happens???