The central feature the Railway Mail Service was the concept of sorting mail en route. This scheme, begun in 1863, saved time and work because the mail in many cases was ready for distribution by letter carriers by the time the trains arrived. Postal officials in large cities were quick to notice the effectiveness of the Railway Mail Service. They began to wonder if something similar could be used successfully in urban areas. Mail carried between sub post offices, or between post offices and rail stations could be sorted on specially designed trolleys or streetcars.
The first such Railway Post Office streetcar was put into service in St. Louis, Missouri, under the direction of that city's postmaster, Major John B. Harlowe. The service began in October 1891, following two months of experimental service.
Brooklyn was the second city to give streetcar RPO's a try. In 1894, the Brooklyn and Coney Island Railway Post Office was inaugurated. Brooklyn's passenger trolley cars had been in service only a few years before postmaster Sullivan decided to follow St. Louis and establish mail service on the trolley lines. Sullivan contracted for special trolley cars to be built so that mail could be sorted as the cars moved from station to station.