I noticed that although there were plenty of shots of the train, there was hardly anything of the tractor, apart from a coupe of the upper part of the cab and, what is probably more important, the skip that it appeared to be towing. I also noticed that the approach to the crossing included a sign with a warning of risk of grounding to such vehicles. Would it not be unreasonable, therefore to consider this as a possible cause. Driver tows the skip across. Grounds it. Gets stuck. Train comes along. Thwack!!