As far as learning anything about the URR, I would suggest obtaining (not free, except at a possible library) railroad books about steel mill railroads. I believe several have been written about the URR, but understand that most of these are just photos of equipment in use.
For steel mill modeling, there is a free (to download) publication called The Mill found at this link:
https://steelindustray.blogspo...s-available-for.html
Also, you can join the group called STEEL at groups.io. Many photos of old steel mills can also be found on the Library of Congress website, and many local historical societies (county and state) have additional photos. There might even be a Facebook group dedicated to steel mill modeling.
Good luck in your search.
Chuck