The federal government considered that anywhere route traveled by a train to be a mail route creating a national network of railroad mail service named, appropriately, Railway Mail Service or RMS. Railroads from the mighty to the tiny held government contract to carry the mail from between cities to the mountain villages of Colorado. Railroads also had fleets of railway post offices, or RPOs, for mail sorting between stations, towns, and cities. Mail sorting in an RPO was hard, fast-paced work with only the most reputable and qualified applicants becoming clerks for the RMS. In this circa 1915 view, we get to see the interior of an RPO and a clerk sorting the mail.
For more information on this fascinating aspect of US Mail and railroading history, click here:
http://en.wikipedia.org/wiki/Railway_Mail_Service
