Mike:
If you look at John Templeton's report of 11/1/56 we can confirm some of this "theoretical" data:
Car #715 @ 21 tons
Car #839 @ 23 tons
Car #860 @ 24 tons
Both the D&S and C&TS use National King Coal grade "RR Nut coal" which would vary from the density that I used for my theoretical calculation based on the specific grade and moisture content. I think John's notation confirms I was in the ball park. The difference in the 700 and 800 loads remember is that extra 6" side board that was added.
BTW King Coal raised the price this past month of their product to $80.00 per ton (was $65.00/ ton for past 4 years). I suppose compared to what we will see in our heating bills this winter it is a bargain...