On July 7, 1838, an act was signed into law designating all railroads in the United States as post routes.  By the late 1860s, the Railway Post Office had become common place on nearly every railroad, with mail clerks riding the rails in specially designed RPO cars for sorting the mail enroute.  The RPO reached it's peak use during World War II.  The introduction of faster transportation means for mail movement, specifically Highway Post Offices and airmail delivery marked the beginning of the end for the RPO.  By the early 1970s, only one RPO route remained.  On June 30, 1977, the era of the Railway Post Office ended officially.

U.S. mail with Scout commemorative stamps, cachets or addresses is possible for collectors to find.  Scout collectors should also look for similar covers in other countries that employed railroad posts, including Canada and the U.K.  Here are two examples.

Occasionally Scout commemorative stamps were used on these covers.  Commercial use covers can be distinguished as railroad post by the initials RPO in the cancel or by a backstamp.

A commemorative cover from the Girl Scout Roundup in Vermont with #1199 first day of use is known with an RPO cancel.