A catcher pouch was a mail bag used by Railway Post Offices of the nineteenth century and the early twentieth century. Its use was limited to exchanges onto moving trains. The specially constructed catcher pouch was grabbed by the catcher mechanism in the passing railway car and the catcher pouch would release from the holding rings on the mail crane. 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.