The late Terry Metcalfe's UP Modeler, Vol 1, shows photos of the consist of the UP Fast Mail in 1957 in Nebraska. Included is SP 5713 box car fitted with steam lines in this case so one might not consider it to be a "plain" box car.
I don't have a roster in front of me, but I think the 5700 series were B-50-24's that were built for express and "Overnight" service. Thus they would have had steam and signal hoses from the get go.
