I think I can shed some,light on the switching of gons from coal to other
services. As late as 1980,when I worked in the BN Chicago regional office,one
of my duties was to pull certain series of true open hoppers(with gravity
gates,not flat bottom,high side gons or tub gons) from unit coal trains to
ballast service in the spring and back to coal in the fall.
As to earlier years gons could be used in scrap,aggregate,sand,etc
service when not needed for coal in the warmer months.
As to leveling or equalizing the coal loads, a high percentage of
coal was the retail trade. Most retail yards were not well capitalized and
certainly wouldn't want to sit on inventory all summer.
I know first hand from my childhood, my parents waited as long as
they could before ordering that first fall coal delivery.
Leo Phillipp