As early as 1865, before the arrival of mail cranes, mail was exchanged on nonstop trains, but to do so, engineers had to slow trains down to a crawl so clerks could exchange the mail by hand. This system, both inefficient and dangerous, was soon scrapped. The first track side Railway Mail Service cranes were wooden, F-shaped, mechanisms. They were soon replaced by a simple steel hook and crane.
As tremendously successful as it was, "mail-on-the-fly" still had its share of glitches. Clerks had to pay special attention to raising the train's catcher arm. If they hoisted it too soon, they risked hitting switch targets, telegraph poles or semaphores which would rip the catcher arm right off the train. Too late, and they would miss an exchange. Each missed exchange netted a clerk five demerits.
Missed exchanges were a special threat on a handful of eastern runs that had less than a minute between some exchanges. On single line tracks, mail cranes could appear on either side, and woe be to the new clerk who, alertly looking out the right-hand side of the train, missed a series of mail cranes on the left-hand side. Experienced clerks on board night mail trains relied on the sound or "feel" of the tracks, knowing by the train's speed or the curves of the track how far away they were from a mail crane.
Exchanging the mail was a two-part process, after the clerk snagged the mail bag with the catcher arm, he had to toss out the mailbag for that station. If a clerk did not kick the mailbag out far enough, it could get trapped beneath the wheels of the train, bursting open and sending letters flying everywhere. The clerks called such small disasters "snowstorms." On the other hand, too much "oomph" could also cause difficulties. One poor clerk tossed the mailbag out with such force that it sailed through the bay window of the station house. Another clerk kicked off his shoe along with the bag.