They have several for sale linked from the article page, for instance a 1989 diner car which I think would have everything you'd need for an office, add a bed, food, bathroom, nice big windows.
They are often railworthy but they may not be up to Amtrak's standards. However, if you find a museum with a locomotive and cars; you can often hire them to run you someplace. But you may be pulled by a freight loco.
Lots of railroad museums will have old private cars, some even road worthy.