There was a time when some Class 1 railroads felt the need to replace their top trains equipment in only 11 years, to be competitive.
Hence both the prewar 1938 streamlined Brroadway Limited, and Twentieth Century Limited got all new equipment in 1949…
If only Amtrak could do that today…
As others noted, this was also in the context of railroads tending to cascade equipment to lower-tier trains on a regular basis. Also, there were significant "gains" to be had on those trains as equipment became lighter and capable of going faster, and where the equipment being replaced on the lower-tier trains didn't have (for example) air conditioning. Some of the reassigned heavyweight cars got re-dressed to look like streamlined cars as well.
The late-40s overhauls were probably also a matter of scale - passenger equipment production got drastically cut back from 1942-46 (I think a limited amount might have been allowed to continue, at least of commuter-type equipment, but there was an approvals process and some already-ordered trains in the early 40s got dumped because of that), so there was a lot of replacement that had been delayed. I suspect that a few railroads just decided to overhaul all of their top-tier trains (and cascade the existing equipment down the line) at the same time and were able to get some discounts from Budd, Pullman-Standard, and ACF for larger orders.
And finally, I'd point out that trains like the Twentieth Century Limited
and Broadway Limited
were in an odd category of their own. IIRC second sections weren't unheard-of, after all, but the fact is also that there were "secondary" top tier trains. I've linked a timetable below, but the idea of cars being dropped back from the Twentieth Century Limited
to the Advance Commodore Vanderbilt
or Commodore Vanderbilt,
or ending up on the Detroiter
or Cleveland Limited
(both of which were all-Pullman no-intermediate-stop expresses) wouldn't have been absurd. Per the timetable below, you had a bank of twelve
trains arriving into Grand Central from west of Buffalo between 0630 and 0930. In that context, overhauling the top trains every decade or so made a lot
of sense in a way that Amtrak just hasn't had the scale to manage.
In short, this wasn't "The New York Central/Pennsylvania Railroad were replacing their equipment every 11 years", it was a whole bunch of circumstances stacking up to make it necessary...and not all of those had to do with just