The cars belonged to the Railway Express Agency (REA) and were staffed by REA personnel. As already mentioned, REA was the forerunner of FedEx and UPS. Depending on what was carried, the agents may even have been armed.
REA also provided fire protection for out-of-the-way roundhouses and other locations where local fire departments were not available. I don't remember who made them, but I have two HO scale REA fire trucks.
Here's the Wikipedia entry about the REA:
http://en.wikipedia.org/wiki/Railway_Express_Agency
You are thinking of the Railway Express Agency...an operation kind of like UPS . They lasted until the mid 1970s.
It would have been a Raiway Express Agency car that went out of business n the late 60s as passenger traiin optona declined..
I seem to remember (not sure if it was on the PC or PRR) passenger trains that stopped in Johnstown, PA having a "green" boxcar type car in there consist. Usually right behind the locos. It was marked for some type of express service. Anyone have any idea what i'm talking about and what it was. It would have been during the late 60's i think. (was was much younger back then)....