Today no one really thinks about air mail as anything particularly unique. In the past, however, the railroads transported most interstate mail in the U.S. To receive a letter with a special air mail envelope and stamp meant that it was either important enough for the sender to pay extra for the postage, or that it came from overseas. The United States Post Office Department, which is today the U.S. Postal Service, began experimenting with air mail delivery as early as 1911. In 1918 the Post