Great photos Borreas, thanks for posting.
This is the first I've heard of the USG being a "government operated" railroad. Last time I was in Plaster city (about 1991) US Gypsum owned the railroad, and ran two trips each day up to the gypsum mines and back. Unless something changed, The US part is just a name.
At that time, there were fences around the unloading facility, which is on the north side of the road, and the locomotive maintenance facility, to the south of the road. That was the total extent of "security" at the place. All you had to do is drive up the dirt road next to the railroad, and you had access to almost all of it. I only drove about 5 miles up, because I was driving a 2-wheel drive conversion van, but I heard the sand got real deep about 10 or 15 miles from plaster city, and I didn't want to push it till I got stuck. The van didn't belong to me.
At the end of each operating day, the locomotive was moved across the road, for storage and servicing. The train was left north of the road on the main line, and there was really no one there watching it. The "handcar shed" and other track maintenance stuff was out in the open about 1/4 of a mile from the main office.
USG was actually very friendly to railfans. This was soon after the Railroad Magazine article came out. I believe Dan Markof has run the Eureka on that road, so I suspect a railfan trip could be aranged, if you really wanted to.
I'll back channel you about my photos from '91. Good luck.