Larry Ostrech writes:

"Tim Gilbert pointed out that the percentage of home road cars staying
on-line greatly increased throughout the depression years, and so the
distribution of cars nationwide was quite different in 1949 than in 1938.
The more-or-less uniform distribution of plain box cars is far more apparent
in the late 1940's than in the late 1930's."

I'm not clear on this. Where does this come from?