I will theorize that the destination had a lot to do with whether the car used was a coal hopper or a gondola. Especially if the receiver wasn't set up to unload by opening the doors below.
Also - don't forget that cement plants use coal for heating and drying the product - so even if you aren't modeling a "coal road" you can still have loads of coal that are going to a cement plant (rather than a power plant). And coal for heating - to small towns - would often be sold or delivered to/from open 'bins'. Neither of these are large volume users when compared to the amount used for power generation or a steel mill - although the amount of coal to a cement plant isn't "trivial". And let's not forget the seasonal variations in the coal traffic.
