Marty McGuirk wrote

If you think the Santa Fe stockcar was an "odd" choice -- wait until you see
the next all-new injection molded car I've got up my sleeve.
I'll send one to Richard to build (he, and about three other people in the
country, all sworn to keep quiet, know what it is), although it's a car that he,
Mike B., and every other UP and ATSF fan have ABSOLUTELY NO use for...

I love a challenge!! :o) Let's see... can't be an SP stock car, since that
would be very useful to UP modelers. (Which reminds me, why the heck didn't
you go a Harriman stock car instead of that obscure southwestern railroad
John Senta Fey or whatever it's called?)... Hmmm, it's got to either be a
car built before 1860 (before either UP or ATSF existed) or a hopper car.
Can't be N&W, since we know those did travel out west on rare occasions.

Aha! It's a two-bay, AAR alternate standard 50 ton hopper.

Ok, maybe not. That doesn't address the ABSOLUTELY no use for...

OOOH! OOOOH!! I know -- it's a MILK CAR!!!

But wait. Many of those ended up in wine service from the west.

Hmmmmm.... I guess it could be a CABOOSE. Phooey.