Tim O'Connor, commenting on the 1938 data [ I think ] notes:
You say your data show "dominance" of SP cars -- but isn't it just
201 or so SP cars in 34 random trains over a period of a month? Now,
that might show "dominance" but it might just also be random luck.
If the UP ran 3 trains a day, I'd say that was a great sample. But
even if UP only ran 20 trains a day... Well, it's not much to go on.
But, Tim, all of the 1938, 1949, 1951, and 1956 books show SP box cars on the UP to be present MUCH more than projected by G-N.