The Inland Empire received its first railway mail service in 1876 via Los Angeles, and a newly completed line to Colton. The railroad was extended into San Bernardino in 1883, and the city received its first direct railway mail service.
In the early 1900s, the Railroad Post Offices expanded into the interurban trolley systems that were developing in many larger U.S. cities. The Pacific Electric Railroad incorporated numerous RPO cars in their vast system across Southern California. RPO service to San Bernardino via the Pacific Electric began in 1914.
Interestingly, postal customers could actually walk up to a stopped Pacific Electric RPO car, and insert a letter into a special mail slot on the side. The Pacific Electric RPO cars carried mail between Los Angeles and San Bernardino up until 1950.
Railway Post Office service peaked in 1915, when 20,000 clerks handled mail on nearly 4,000 RPO cars rolling over 216,000 miles of track.