Where did all the passenger cars go that were owned by railroads after Amtrak started in 1971? I know they didn’t buy all the cars. Autotrain bought some, some were scrapped (P70 coaches, B&O six wheel cars, etc.). sold to commuter railroads, etc.
A lot of the cars were scrapped - especially the Pullman-built cars which were more prone to rust-out compared to the Budd-built equipment. Many of the railroads had cascaded former mainline cars into suburban or work service as the train-offs mounted. This was true even before Amtrak.
Metro North bought a lot of long distance coaches and replaced all ex-NYC heavyweights with lightweights and installed suburban high density seating. I believe some other commuter authorities did the same, some in Boston suburban service. The Long Island replaced all its ex-heavyweight six-wheel truck PRR parlor cars used in weekend Montauk and Greenpoint service with lighweight sleepers and lounges and diners used as parlor cars. The ex-Pheobe Snow observation (Budd) pair used as Metro North official cars were part of this fleet on the LIRR for a time.
There were auctions for cars in St. Louis quite often in the 1979-1983 time period when I lived there. I presume there were similar sales elsewhere. I kept meaning to bid on them (they sold anywhere from $200 to $6000 for a baggage to dome observation respectively), but could never find any place to “park” them. Union Station “parking” was $250 per night… If I had only discovered the Rocky Mountain****nal which was storing for $1 a night at the time.