A guess.....

As a retired railwayman who used to be involved in things like reporting marks and car service rules, this is very interesting.

Clearly some references indicate USGX reporting marks are for Holly Sugar. In Googling Holly Sugar you eventually figure out it was acquired by Imperial Sugar and probably no longer exists except perhaps as a brand name. The sugar industry has gone through a series of mergers (and now apparently spin offs), with Imperial going through bankruptcy in the process.

Some other references show the USGX marks belonging to U.S. Gypsum. Googling USG pretty quickly shows there is no connection between USG and any sugar company.

So I would GUESS that the USGX marks at one time belonged to Holly, perhaps came from Union Sugar, but when the sugar business merged, downsize, went bankrupt, or whatever, the marks were abandoned. But some older references never got changed.

So perhaps after their abandonment by Union/Holly/Imperial Sugar, USGX got "ownership" of the marks, since they are certainly logical marks for USG.

But what is also interesting is the only USG railroad I am in anyway familiar with is the narrow gauge line at Plaster City, in southern California (although there may be more). Since that line is ng. it does not interchange cars with other lines, and therefore has no real need for AAR reporting marks. On the other hand, perhaps USG owns or leases some standard gauge cars that are in interchange service, and simply uses the AAR marks on all its rail equipment whether in interchange or not.

The car in the picture certainly looks narrow gauge to me. I wonder if anyone can do a trace on the EJ&E 21 flatcar it is on to find the destination. I'd guess it is most likely Plaster City, CA.