In the 60's and 70's the Southern regularly ran piggybacks with their passenger trains. It wasn't uncommon to see a four or five car passenger train with 10-15 TOFCs on the end. I believe Southern charged the shippers a premium to run them on a passenger schedule. This was one of the ways Southern tried to make passenger service show a profit instead of a loss.
Gunn shot the express cars off Amtrak.
Picture worth 1000 words department: Here is "The Ann Rutledge" at Hermann, MO in the days when it still carried express MHC cars. Note how they are on the rear of the consist.
The car in the rear is an express boxcar used by railroad Express, the railroads version of UPS. These cars were especially outfitted with steam lines and high speed trucks to run in passenger service.
FEC (Florida East Coast) will haul a passenger car behind their freight train if the person or company will pay for it, but you must have your own water, electric, and cooling & heating systems as FEC won't provide that for you because of being at the rear of their freight train. I have actually seen FEC haul a passenger car from Miami FL to Jacksonville FL behind a freight train.
Lee F.
In this case the passenger cars on the head end may be crew quarters for the transcontinental freight haul. Seen at Toodyay Western Australia in 2009.
The lead unit happens to have a passenger train paint scheme for an entirely different route.
Note the 3-rail track.