There are FRA rules, AAR rules (for interchange) and for passenger cars, Amtrak rules...

Currently, per FRA rules, for operation on a FRA regulated railroad, any "Freight" cars (including cabooses) need a waiver for operation if over 50 years old. ) Passenger cars do not have a similar rule and are legal if mandated maintenance and inspections have been preformed.

I have three freight cars under waiver, and that waiver limits those cars to 20 mph, and requires a "comprehensive" inspection every two years.  That calls for lifting the cars off their trucks, inspecting the frames, draft gear and center plates, and a single car air test.

I have 5 passenger cars in service, ranging from 97 years old to 70 years old.  They need the single car air test but don't require a waiver or a more detailed inspection for legal operation.

Randy Hees
Nevada State Railroad Museum, Boulder City
Nevada Southern Railroad