Many of the private railcars in existence today are ex-Amtrak, because when Amtrak was formed on May 1, 1971, they assumed most of the passenger rolling stock of the railroads that discharged passenger service.

Supposedly the biggest cost associated with refurbishing a car is passing the Amtrak inspection of the bogies, axles, and wheels -- I believe they have to be completely replaced at 40 years of age. When a railroad retires a car, this equipment may be close to end-of-life.

Many freight railroads still have "office-car specials" -- passenger equipment for the executives to use. Those would probably be the easiest to get up to Amtrak specs.

Trivia: Amtrak will only run trains with private railcars at a maximum speed of 110 mph. On the Northeast Corridor, private railcars are only allowed to be added to the overnight Northeast Regional train which normally travels at 125 mph -- but there's enough padding in the schedule that they can slow it down to 110 without affecting the timetable.