Your guess is as good as mine... Commodities like coal and grain are volitile animals that highly compete for equipment. There are suits in court under litigation against the railroad and other industries, filed by grain consortiums, trying to address the seemingly unfair allotment of equipment present in the rail industry. If you do a quick search on "railcars" you will undoubtedly uncover several articles to the same.

Railcar availiblity was always a concern for me when I was shipping material by rail. There were occasions where I could not get equipment on a timely basis, and when I did, there was no guarantee the railroad would pull them and place them when the customer needed. My industry was minor compared to coal and other commodities. Demurrage was high if a car sat too long, and would happen from time to time due to the railroad flooding me with cars. It was feast or famine. In central texas, it was somewhat better than Tennessee or Alabama, but I had to load 50 cars as a unit, and had 48 hours to complete loading (or face seriously high demurrage and other fines).
This cycle would replay 10 or so times per month. Customers that used to purchase 10-15 carloads were cast aside as the rr would not break the train, and those customers could not accept 50-car units. If only we as a nation had subsidized the rr to the extent we did trucking... What's that, you say? How did we subsidize trucking? We built ROADS everywhere...

__________________
RM Smith
1988 560SL

"Where is it again that we are going, and why are we in this handbasket"?