With the establishment of the first air-mail route in 1918

1918 in aviation

This is a list of aviation-related events from 1918:- Events :*The Imperial Japanese Navy establishes its first lighter-than-air aviation unit.* The naval aviation branch of the Chilean Armys air corps receives its first aircraft....

, and the later additional routes, plus the accepted use of premium priced air mail by the public, it was only natural that the Railway Mail Service

Railway Mail Service

The United States Postal Service's Railway Mail Service was a significant mail transportation service in the US during the time period from the mid-19th century until the mid-20th century. The RMS, or its successor the Postal Transportation Service , carried the vast majority of letters and...

 (RMS), being in charge of transit mail, was assigned the task of establishing Air Mail Field (AMF) postal facilities at the major airports. Only outgoing air mail was distributed at these workrooms, channeled there by both the post offices and Railway Post Office

Railway post office

In the United States a railway post office, commonly abbreviated as RPO, was a railroad car that was normally operated in passenger service as a means to sort mail en route, in order to speed delivery. The RPO was staffed by highly trained Railway Mail Service postal clerks, and was off-limits to...

 (RPO) routes. This mail was distributed and dispatched to other AMFs via the different flight connections. Incoming mail from other AMFs was distributed by general scheme and pouched to outgoing RPOs and necessary post offices. The Motor Vehicle Service provided frequent trips between the city post office and the AMF for air mail ground transportation. In 1951, there were 1200 clerks manning forty AMFs. They continue to exist within the U.S. Postal Service (USPS) and are now called Air Mail Centers (AMCs).