I don't know anything about the UP operations but the IC rostered a Banana Messenger caboose as late as 1960.  It's my understanding that produce buyers rode the trains. 

The IC also ran strawberry trains known as the Crimson Flyer and they often used an old coach instead of a caboose to carry the produce buyers.   Apparently all of the crop wouldn't be sold when it left Louisiana and the buyers would take orders by telegraph and cars dropped off at interchange points.  According to an article in the Green Diamond (IC Historical Society Magazine) they used REA Express reefers on the Crimson Flyer runs.

I am assuming the "banana messengers" performed the same function with all car loads not being sold when the trains left New Orleans and cars that were sold enroute set out at East/West interchanges.