the main thing driving larger cars was cost, not revenues.
revenue was less important because rates were regulated on
the basis of tons and distance, so the number of cars made
a bigger difference to railroads than to the shippers. the
"big john" rate battle in the 1960's was fought over the right
of the railroad to offer a lower price based on a larger car --
to the ICC this was 'unfair' and so the battle was joined...
and it took another 20+ years to dismantle the ICC control of
so anyway, covered hoppers of various sizes were used for grains,
cement, lime, sand, etc more or less whatever car size happened
to be available and allowable in the 1950's. it wasn't until the
era of price based on car size (and later, train size) that optimal
loading (max tons per car) became vitally important to shippers.