I suspect there are several answers about the "cause". First of all, US track is horrid, sort of 1870 sort of stuff. The rest of the world installs concrete sleepers (ties). Wood is just not strong enough for these loads. This turns out to be "gauge" issues, like the 4 ft 8.5 inch distance becomes 4 ft 10 inches, or something. Second, these trains are very long, 120 cars. Each coupler has a loose fit, like one inch between open and closed. So, in a 120 car train the train is 120 inches longer when it is being pulled, compared to if it were being pushed. The forces that can happen are huge, large enough to tear apart the tank cars. And then the stuff they ship is a viscous goo, sort of black tar. If this gets cold, it will only flow very slowly from the tank. So the "oils" add gases, like propane, to dissolve in the ooze and get the viscosity lowered. Pictures show the propane exploding pretty clearly. So, ancient, obsolete rail system, very long trains that, if not very carefully managed, create shock waves that break the tanks, and very flammable additives needed to make the stuff shippable. During "start" the back set of diesel locos are supposed to "close the train" by shoving from the rear, and then, when closed, the front engines chug. Very tricky sort of engine driving. In all fairness, BNSF is upgrading track, but there is very much track to upgrade. To be sure, they will do the "main line" first. Sort of a long answer. Bet those guys at UPS could come up with something.