The criminals targeted the mail car in the attempted robbery of Train 13 of the Southern Pacific Railroad on October 11, 1923, because both mail and freight cars frequently carried valuables such as payrolls, cash, gold and negotiable bonds. This 1925 photo shows mail sacks prepared for transport on a Railway Post Office car. (National Postal Museum, A.2006-59)
https://postalmuseum.si.edu/exhibition/tragedy-at-tunnel-13-exhibition/the-context