Since WW1 had started, it could be the USRA was upset, the the Southern boxcars weren't big enough to load war supplies , when they were unloaded on other railroads.
suppose they shipped cotton bales to textile plants in New England . The textile plant might have to use another rr boxcar, to ship out unitforms to a military warehouse,
resulting in at least one extra switch, and the empty southern car having to move empty back to the Southern..
It might be possible to check the official railway equipment register or other sources to see the boxcar capacity of other railroads in comparison.