I'll be presenting on the subject at Collinsville and do not care to give too much away however will say the following:
1) The consignee has always been allowed to specify what type of car they want the shipment made in. I have copies of letter internal to my railroads traffic department regarding where shipments were refused because they came in the wrong car.
2) Commenting on the original comment "small amount of coal", a 1x12 can be placed in a boxcar on edge and several grades of coal shipped, all within one car. With gons or hoppers, sheets of plywood might be used to separate grades of coal ordered.
3) A boxcar gave protection from the weather. Coal is always slacking or put another way, generating heat as a byproduct of oxygenation. "Losing its calorific value" as David wrote prior and even with gasoline this happens as there are products today to help store gas in lawnmowers, snowblowers. (That slacking was how many railroad wooden coal towers burnt. The coal fines became compacted at the bottom of the bins and in some cases, spontaneously starting burning when sufficient heat was generated.)
In a lessor manner, if snow fell on the coal it could be melted in initial contact with the coal, then refreeze as more snow covered the coal and the BTUs needed to melt to ice now formed then overcame the meager BTUs the slacking produced. Another internal telegram sent by a very angry roundhouse foreman related how several men with long metal spikes were needed for a full shift to break the frozen coal away from a hoppers discharge chutes after this had happened at a roundhouse powerplant.
4) Theft prevention, the equivalent of a modern gas filling station drive away.
Jim Dick - Roseville, MN