One good reason to use a circa-1960 cutoff is that the world of freight
cars changed GREATLY around that time: gigantic tank cars, 60-ft. box cars,
85-ft. pig flats, Center Flow covered hoppers, etc

Hmmph. You might as sell say let's stop with 50 foot cars, plug doors, or
10'6" inside height cars, or frameless tank cars. I think Tom's suggestion of
a Rubber Ruler is good enough for me. If a discussion drifts into the 1960's,
as long as there is a connection (however tenuous) to the 1950's, then so be
it. As for the poor souls stuck in 1947, well, they're beyond hope! ;o}