Others have pointed out that there are a few more accurate single
sheathed box car models in HO scale than Gene suggests. However, his
point remains true that, aside from resin kits, there aren't enough
to accurately represent the typical mix of box cars in a steam era
freight car fleet. One reason, of course, is that, apart from the
USRA box cars, there was so little standardization of design in the
teens and '20s. Even the ARA "standard" single sheathed box car was
built with so many different heights, ends, roofs, underframes, etc.
that kit/RTR manufacturers can't find a single version which can be
accurately modeled for more than one or two prototype railroads.
It's not that the manufacturers have overlooked all those single
sheathed prototypes but that they haven't been able to figure out how
to develop models that will sell in large enough numbers to cover
their tooling costs, never mind turning a modest profit. While those
of us who have the time and skills to build resin kits now have a lot
of options, there's no solution in sight for modelers who don't.

Richard Hendrickson