In 1898 Brooklyn and its trolley lines were incorporated into New York City. Most cities used numbers to designate their mail trolley lines. New York and Philadelphia’s postmasters used letters or alpha-numeric systems.
The success of the service encouraged postmasters in other cities to follow suit, even if their city’s trolley service was rather limited. San Francisco and Rochester, NY began operating RPO trolley cars in 1896 followed by Baltimore in 1897. A handful of cities followed, Pittsburgh in 1898, Seattle in 1903 and Cleveland in 1908.