I have one of these, I think it's a Weaver car painted for Seaboard, #722, a 40-foot car.
From what I've read, REA carried things that the RRs did not want to carry in regular boxcars, or things that had to get to a destination in a certain period of time (time sensitive).
Were these Express cars always part of passenger trains or did they tack them to whatever fast freight happened to be coming into town? I can't see them putting them on a slow freight or a local, nor can I see them stopping a fast freight to add in the car, but I don't know for sure.
The real Seaboard cars had high speed trucks, steam lines, and vents on the sides. Seaboard also had 50 foot refrigerated express cars. One commodity they carried was ice cream