"Starting in the 1880s, there were trains designed as "Fast Mails" which were scheduled with the primary purpose of moving the mails (as opposed to moving passengers). These Fast Mails made limited stops in most cases. Some cancels are known from some lines that do have "F. Mail" or "F.M." in the cancel. In my own collecting area, these are known from the Charlotte & Atlanta and Washington & Charlotte. There are others. These designations generally died out by the start of the 20th century. There were still primarily mail trains, but the only indication would be in the Train number. Knowledge of the train schedules is required to pick them out. One example is Train 98 (of song fame) of the Washington & Greensboro and Greensboro & Atlanta."
http://www.philamercury.com/board.p...el&sa=1&pg=2