One railroaders PoV from the AAR manual of standards and recommended practices, 100 ton trucks are used on cars/vehicles that carry 286,000 up to 315,000 lb capacity or 39 tons/axle max. The weight of the empty car plus the weight of the load is the determining factor on which truck to use. Every car is designed to carry a weight that either maxes out in the cu. ft. of space available for the load. or maxes out in weight alone. Those are industry standards (Rules) to be applied when cars are in unlimited interchange service. If cars are in captive service and no interchange those rules can also be waived by agreement.
All in all, sounds a lot like what we've already surmised. 
