The Railway Mail Service of the United States Post Office Department was a significant mail transportation service in the US from the mid-19th century until the mid-20th century. The RMS, or its successor the Postal Transportation Service (PTS), carried the vast majority of letters and packages mailed in the United States from the 1890s until the 1960s. History George B. Armstrong, manager of the Chicago Post Office, is generally credited with being the founder of the concept of en route mail sorting aboard trains which became the Railway Mail Service. Mail had been carried in locked pouches aboard trains prior to Armstrong's involvement with the system, but there had been no organized system of sorting mail en route, to have mail prepared for delivery when the mail pouches reached their destination city. In response to Armstrong's request to experiment with the concept, the first railway post office (RPO) began operating on the Chicago and North Western Railway between Chicago and Clinton, Iowa, on August 28, 1864. The concept was successful, and was expanded to other railroads operating from Chicago, including the Chicago, Burlington and Quincy, Chicago and Rock Island, Pennsylvania and the Erie. By 1869 when the Railway Mail Service was officially inaugurated, the system had expanded to virtually all of the major railroads of the United States, and the country was divided into six operating divisions. A superintendent was over each division, all under the direction of George B. Armstrong, who had been summoned from Chicago to Washington, D.C. to become general superintendent of the postal railway service. Armstrong served only two years as general superintendent before resigning because of failing health. He died in Chicago on May 5, 1871, two days after his resignation. Armstrong's successor in Chicago, George Bangs, was appointed as the second general superintendent of the postal railway service. Bangs encouraged the use of fast mail trains, trains made up entirely of mail cars, traveling on expedited schedules designed to accommodate the needs of the Post Office rather than the needs of the traveling public. In 1890, 5,800 postal railway clerks provided service over of railroad. By 1907, over 14,000 clerks were providing service over of railroad. When the post office began handling parcel post in 1913, Terminal Railway Post Office operations were established in major cities by the RMS to handle the large increase in mail volume. The Railway Mail Service reached its peak