This topic is off discussed and while a very narrow sampling, might provide some info on coal hoppers off-line.  The Interstate was a coal mine short line in the most western part of Virginia.  If you counted the amount of coal it gave the Southern Railway, it would have been Southern's largest shipper.  My info is from Ed Wolfe's "Memories of an Appalachian Shortline"  A chapter from Hank Stuart, Jr.  who eventually became General Manager.
"The cars of the Interstate were protected by the ICC.  Lines in the southeast were prohibited from appropriating Interstate hopper cars for use at mines or quarries."  "While most of the loaded cars stayed in the South where the order was effective, some went north where there was no car order to protect them from appropriation."  Elsewhere, Stuart mentioned the B&O was the worse with one Interstate car staying on the B&O for almost a year.
In 1947, off line Mean times for the 10 largest foreign roads were ACL -13 days.  C&O -13. Pere Marquette-17. Clinchfield-5.  ET&WNC-5.  Lancaster & Chester-10.  N&W-7.  Piedmont & Northern-7.  SAL-11.  Southern-8 and Virginian 12 days.  This table included at least one Interstate coal hopper going to Canada.  The Wolfe books also show a Lehigh Valley twin coal hopper on the Interstate in the early 1960's. Hopefully entertaining.

George Courtney