Inspection Trains (was: Passenger Car Stash)

I noticed that all the major freight railroads seem to have their secret stash of passenger cars and I was wondering.

Do they use these to inspect the lines or do they really travel in these?
How often do they get used?

The main one I know is CSX uses their secret stash to run the Santa train.

There are a number of reasons that the freight railroads have some
passenger cars still on their property. They use these cars for passenger
trains for their high officials for inspection trips, for special trains for big
shippers, employee appreciation specials and other public relations
purposes.
Some of the freight railroads like the Union Pacific have a whole fleet of
passenger cars and others own just enough for a single train.
Occasionally one freight railroad will loan one or more of their passenger
cars to another freight railroad for a reason too.
If one were to lump all of the freight railroad owned passenger cars
together, I think there would be a pretty good size fleet of various and
extremely interesting cars.
Noel Weaver

That would be an interesting project for some one with a lot of time. List all exisitng passenger cars on freight RR's.