The United States Postal Service's Railway Mail Service was a significant mail transportation service in the US during the time period 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. 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 quickly seen as successful, and was expanded to other railroads operating out of 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.