According to the GCOR (which practically all U.S. common carrier railroads follow), the handbrakes are supposed to be set on unattended cars at all times regardless of whether they are coupled to an engine or not and the wheels are to be blocked if the handbrakes aren't enough to keep the car(s) from moving.

It seems more like this may have been another switching accident, though; the switching crew may have been doing some "gravity switching" or were "kicking" the car and the "rail worker" thought the switch was lined properly when it really wasn't. I'd love to know why the dispatcher was "thinking the runaway car was a train scheduled to depart", though, unless a train was leaving the yard on the same track that the car rolled onto; it doesn't surprise me that a TV station would leave details like that out, since no one there and very few (if any) of its viewers would understand such things anyway.