If you have Tony's SP book volume 3, look on pages 213, 214, 225,
and 249 for good shots of lumber loads on flat cars with bracing.
It is my understanding that the best quality lumber went into
boxcars. While flat cars would have carried larger sizes, they also
were used for lumber that was of lesser sizes but with a lower
grade. All of the above photos seem to show this sort of load.