A catcher pouch was a unique mail bag used only by the Railway Post Office of the late nineteenth century and the early twentieth century. The specially constructed catcher pouch was grabbed by the catcher arm in the passing railway car^[1] and the catcher pouch would release from the S rings on the mail crane.^[2][3] This technique was known as "mail on-the-fly". Starting in the 1870s the use of this technique of the Railway Mail Service was an important issue in the United States. It was a popular technique and the backbone of the United States Postal Service through the 1930s.^[4]

When the mail clerk would grab the catcher pouch on the mail crane he would at the same time kick out of the Railway Post Office car the outgoing mail for delivery to that local. The idea behind the catcher pouch was that there could be an exchange of mail to villages too small to justify the train stopping. The complete transfer technique (tossing out the outgoing mail a second before grabbing the catcher pouch) required much skill and potentially could cause harm or even death for those not trained properly.^[5] Another reason why the catcher pouch and mail crane were developed is so the train did not have to slow down just for the exchange of mail (Old School method).^[4]

The mail on-the-fly was not a smooth operating technique.^[6] One of the problems associated with the technique was that the postal clerk had to pay close attention when he raised the train's catcher arm. If it was raised too soon there was a chance of hitting and destroying switch targets, telegraph poles, and railway semaphore signals and in the process also destroying the train's mail catcher arm. If he raised the train's catcher arm to late he could miss the catcher pouch altogether.^[7]

Catcher pouches could not be used for any other purpose. The catcher pouch was to be used only for letters (sometimes newspapers were an exception). The maximum weight of a filled catcher pouch was to be 50 pounds. The catcher pouch was to be locked and placed upside down on the mail crane no sooner that 10 minutes of the scheduled arrival of the Mail Train. The catcher pouch was to be tied in the middle before it was to be transferred. If a small amount of mail, it should be put in the lower half below the tie strap. If a large amount of mail, it should be divided equally between the upper half and the lower half of the catcher pouch. ^[8]

The catcher pouch was made of an extra tough canvas material and had medal rings on each end so they could attach to the arm of a railway mail bag crane. Its canvas body was strengthened by leather bindings around the top and bottom. A Registered Mail pouch came also with a leather bottom and had a special postal lock to secure the contents. A leather strap was secured around the center of the canvas body of the catcher pouch when it was readied to be snatched by a passing train's mail hook.^[9]

• Melius, Louis (1917). The American postal service: history of the postal service from the earliest times. The American system described with full details of operation. Washington, D.C.: National Capital Press. ((cite book)): Unknown parameter |acccessdate= ignored (|access-date= suggested) (help) at Internet Archive

• mail bag
• mail pouch
• mail sack
• mail satchel
• portmanteau
• mochila

Wikimedia Commons has media related to mail bags.

• Mail on-the-fly 1903 video from Library of Congress Notice the clerk tossing out the out-going mail a second before catching the catcher pouch.