Sugar beet service required any available car at harvest season, old or new. On the Q this meant late summer early fall. On the SP sugar beets were almost year round, which is why the SP had gons dedicated to beet service. The Q did not.
Coal was seasonal, as Leo points out retail trade was often heaviest in the fall through winter, then slacked off spring and summer. Remember also that coal if wet could freeze in the winter months, esp up north, so boxcars would be used during the winter.
Gons were preferred for coal traffic on most western roads for two reasons, the line haul was long, so return loads were sought out. An empty gon makes no money. A gon with a load of sand, stone, steel, lumber, actually anything that could go on a flat car, was a car making money. And two, when coal traffic slacked off the cars could be used for other loads, no strings of empties plugging up yards. A mine work stoppage, a strike, seasonal downturns would mean unused cars. Better to have them hauling something than nothing.
Doug Harding
www.iowacentralrr.org