Doug Harding writes:
"What is a UP HK-50-4?"
Well, of course, a UP HK-50-4 [ as well as the similar HK-50-3 and HK-50-5 ] were ballast hoppers. The HK-50-5 might actually be a better choice because it was originally built as a composite car with wood siding which was changed to steel in '51. The car is perhaps more interesting because it has outside bracing. The HK-50-3 and HK-50-4 had straight "slab-like" sides which only a UP nu...uh...modeler would probably appreciate.
"And until now, I was not aware I needed one."
Well, I just naturally assumed that most RRs [ well...maybe not those in New England...wherever that is...] would have bought large amounts of ballast from Buford [ on Sherman Hill ]...or even from Bruceford...that is, if the Fast Mail ever got out of the way. Anyhow, long strings of ballast cars could generally be seen moving across Big Wyoming. What more could you want?
Lessee, Doug. Don't you model the M&StL? You might want to consider the tank car.