Hi,
When I wrote my term paper on the Gainesville Midland c1975-6, I knew that the GM only ran in GA.
I went down to the State Archives and looked up the Public Service Commission / Railroad Commission annual reports.
It was quite interesting seeing the different types of loads carried and where they were destined for.
One interesting item was the that coal was a reported in large quantities for a railroad that ran through piedmont cotton fields. Between 1906 and 1915, the line was dual gauge. A dealer (or dealers) in Gainesville GA received a SG load of coal. Thenhe would break the load up and reship by NG to the various NG only locations. So one load of coal got listed 3 times - delivery to Gainesville GA in SG cars, unload SG: load NG: from Gainesville in NG cars to (for instance) Bethlehem GA :unload NG.
I know SRR and SAL also had reports in the GA PSC. I've never looked but there may be interesting data like that for the agency in NM and CO similar to the GAs PSC.
Doug vV