Over the years, I have followed some folks and groups who have rebuilt their passenger rail cars to Amtrak operating standards.
Here are a few cars that I follow.
One car that I really like is the NKP 1. It was built for the Nickel Plate Railroad in 1929 and used by the Van Sweringen brothers.
I'm working on a post to explain what it takes(besides money) to get a private car up to Amtrak standards.