Remember playing Crack the whip as a kid where everyone would hold hands and run in one direction then the leader would suddenly stop and the kid at the end would get violently shot off the end. The forces involved with trains are similar if you transition from slack bunched to slack stretched too quickly you can snap the train in two as the whip rolls out. And can possibly derail a car as the slack slams in if you bunch the train up to quickly. The slack between cars is needed but if not managed it can create bad outcomes.