We have many small "windows" into the past...of different types from photos
I do know MP and Southern hoppers came into Pueblo on the MP. During
the beet season open hoppers were in great demand and you could see a
real mish-mosh of different owners' cars loaded with beets. And coal
was loaded on the 'Colorado & Wyoming' branch out of Laramie; I don't
know if any of it was sent to CF&I in Pueblo. I'm not speculating;
just suggesting a possible routing to look up as a way that MP (or
Southern, or N&W) hoppers may have ended up in Laramie...
As for Tony's idea of MP cars in Utah, I have to say I'm amazed how
hard it is to find photos of the diverse cars that could be found at
the USS Geneva steel works in Provo. Many parts of the mill and the
freight yards were very visible from public roads. Freight car fans
are just a tiny minority of the railfanning public, I guess.