In the absence of a real Rio Grande expert, I'll throw in what I know.
I'm modeling an area of central Kansas that had a direct connection to the Rio Grande via the interchange at Pueblo. By the late sixties, for sure, there was no coal traffic coming in my direction from Colorado, or any other mineral traffic to speak of, coming from that direction, so I never saw any of these cars there, and they were probably less likely to have strayed farther eastward. I believe these cars carried coal and limestone, mostly to CF&I at Pueblo, and mostly coming down the grades on Marshall Pass, after they got rid of that pesky narrow gauge. That's probably why they also have clasp-brake trucks.
Still, for the 46-footers for sure, this is an extremely neat car not likely to be made any other way so, if I could use one, I'd be all over this. Ditto the heavy-duty flatcars made out of tenders, which I didn't know existed. The MoPac did something like this also, but with two tender frames welded back-to-back, and I'd love to see somebody who has the skill draw and build those.
I think we're on the cusp of a big development, making 3D printed freight cars of unusual prototypes, and I'm hoping to encourage it. The only thing I'm not satisfied with is how hard these things are to find, which I acknowledge has nothing to do with the skill of the designers of these cars. I tried hunting for that link last night and completely failed, so thanks for posting it.