Interesting question Dan
Are you talking standard or narrow gauge?
Standard Gauge:
The USAT 'Ultimate' (and their 'ordinary models) are supposed to be 1:29 scale and most of the LGB box cars are 1:22.5 (allowing for the LGB rubber ruler of course).
I have run both in a rake and they look okay as, in real-life, different makes of reefers/box cars had a slight difference in height any way. Having said that the LGB 40ft cars look best with 50ft (or larger) USAT ones.
Narrow Gauge:
USAT has the old "American' range at 1:24 and LGB older style ones seem to be a similar scale as they are pretty close in dimensions.
All in all, whilst there is a discrepancy in scale and sizing, it does look okay, depending on what the prototype that the cars are modelled on...... oh yes....and you squint your eyes....