I got this info from Jerry Day on why coal was put into box cars--quite interesting:
The local domestic coal dealers were small operators and some did not have warehouses or places to store the coal. They would have the D&RGW put the car on a siding and sell the coal directly from the box car. They would have to pay the D&RGW for the time the car sat there, but cheaper than building a place to store the coal.
William
aka drgwk37