Truly rusted out looking, bad paint, illegible lettering was
-relatively- rare in the 1950's but was not unheard of -- and
a car that is only a few years old could get pretty cruddy in
the steam era. Yes, the bulk of box cars and reefers should
merely be "dirty" to various degrees, with reweigh stencils
and chalk marks being pretty common. In the 15 years after
WWII I think something like 1 MILLION freight cars were built
and other hundreds of thousands were repainted or rebuilt.
So in my opinion shiny NEW cars are under-represented on
most layouts. Every now and then just give one car a dip in
a bucket of Future. That should do the trick. Of course, the
date on the shiny car will pretty much fix the exact era of your
layout and the Prototype Police will put that information to good
use in their unrelenting quest to stamp out anachronisms!

Tim O'Connor