what did I see

Just Joined the group today, hoping someone will shed some light on what I saw the other day.
I caught an EJ&E 661 eastbound out of Joliet, IL with this freshly painted blue flat car (see link below)with reporting marks of USGX 15 Works, I have been told on another board that it's a narrow gauge, which makes sense after I looked at it on top of the other flat, but there's was some discussion on the reporting marks, is it union sugar? us gypsum? and it was going eastbound, I am assuming it might be going on some sort of static display...an info would be helpful

Thanks
Bill Grenchik


[www.rrpicturearchives.net]