The Interstate Railroad, despite its name, only operated in Southwest Virginia. It interchanged with Southern (which acquired it in 1960), Clinchfied, Louisvile and Nashville, and Norfolk and Western.
Two good books on this coal-hauler are:
The Interstate Railroad: History of an Appalachian Coal Hauler (1994) by Ed Wolfe -- It has a list of rolling stock (mostly hopper cars) starting on page 146. Hoppers were mostly 50 and 55 ton cars.
Applaachian Coal Hauler: Interstate Railroad's Mine Runs and Coal Trains (2001) by Hugh Wolfe and Ed Wolfe - page 145 ff. deal with car supply and car rebuilding, and how per diem charges affected operations.
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