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.
- Jim B.