Courtesy of Michael Bartolic, here is a link to a photo of PFE reefers being used to handle the Christmas mail rush:
Caption: “A long string of Pacific Fruit Express reefers is being loaded with mail at right as workers sort mail bags at the Union Pacific Transfer in Council Bluffs, Iowa, during the 1951 Christmas rush. At busy times like this, all available equipment was pressed into mail service.”
Santa Fe reefers also saw similar use.
Railroad Citrus Industry Modeling Group