I expect coal shipments in gons were quite common into at least the 1920s. Although the term sounds a bit “toy train” to us nowadays, multiple railroads listed coal carrying (usually drop-bottom cars?) gondolas as “coal cars”. A quick look at the March, 1924 RER list cars by that description as CofG 14500-14600, then cars in series 15250-16049 and 17001-17200 as “coal gondolas”. The A&WP lists cars as “coal, solid or drop bottom” in seven or eight car series with a few “self clearing” coal (sic hoppers?) at the end of the RER entry. Examples of “coal cars” continue through the RER.

As most freight cars were more or less free running, Could we assume that gons loaded with coal would be common in interchange service?

On the Southern, early steel cars used for coal did not have particularly long service lives. Part of the reason may have been the first groups (incl the USRA examples) were about 20 years old during the depression and were simply not needed. Was steel produced in the teens and early twenties less able to deal with the corrosive effects of coal loads than more modern steel?