I realize that the Accurail USRA hoppers are not prototypically correct
for the IRR in 1951, but I can find no prototypically correct hopper for
them for that era, and the Accurail is my stand-in until either I or some
manufacturer can remedy that. Any suggestions for something closer?

No easy ones. Interstate had three main groups of hoppers over the
years:

2000-2799, Gla clones built in 1913 and 1917;
5000-7009, one-foot-taller Gla clones built in 1920 and 1923;
7010-9120, N&W-design, class H-6 built new for INT by Roanoke
(7010-8009, 1944-45), VB&I (8262-8761, 1947), and Interstate
itself (1947-52)

The earlier groups were scrapped gradually in the late 1940s and early
1950s, giving their trucks to the H-6 cars as they were built. The Gla's
can be modeled with the Bowser car, but those were practically extinct
in 1951. The foot-taller cars can be kitbashed from the Bowser, but they
didn't last much longer than their shorter brethren. The H-6 is mostly
a flat-end version of the N&W H-5 and is within reach of a heavy kitbash
of the ECW H-2a kit.

The Accurail car can be used to model the USRA clone hoppers Interstate
leased from Koppers in 1924-1929, and the secondhand cars INT bought in
1956 (class HM3, which I think makes them ex-NC&StL cars).

David Thompson