The nice ones are almost all old business cars. The business car was used by the railroads for senior executives to move around their systems, and hold meetings.. usually contain a couple of executive bedrooms, a staff bedroom (they typically carried a cook and a steward, although the roles were sometimes combined). The rear half or so of the car is an open plan lounge/meeting room.

The cars were usually built by a company like Pullman, usually from a time frame of roughly 1900 +/- 20 years.

Huge money pits, with tons of (often quite ornate) wood m, etc. then add the cost of restoration (again almost all of these cars are 100+ years old), retrofitting modern electrical systems, air conditioning. Could easily be a million dollar project.

so basically all the problems of buying a castle or old chateau, but on wheels.

Kind of. You don't have to worry about foundations, or sewer lines or frost heaves or masonry. So there are advantages.

But I mean, just look what a nice one is like inside.

Something like this one (Which I've actually been in)

https://www.youtube.com/watch?v=xJ9TscDfHMQ

More like an old Winnebago that wasn't built to a price point (which is both good and bad)

Not really, you just need to get more people. The fanciest car holds 8, other cars hold 20 to 70 people. So if you divide the price the people it's not that bad.

The first time I realized this kind of thing was a tour of a baseball stadium. They showed us the suites. I forgot how much they cost but if you got a bunch of friends together to fill one then they were in the same range as medium good seats.

Example: https://www.mlb.com/padres/tickets/premium/suites Various prices, one is $4260 for 20 people. That's $213 each. Is that rich person's thing?

It wasn't clear what the private car costs but, just guessing the Train Jam did this. https://trainjam.com/faq You can see the prices for 52hr ride.

Every rail car is a private rail car, if "shared with 20 to 70 people" is your definition of private :)

If the 20 people are close family then …. Yeah?