"The United States Postal ServiceĀ“s Railway Mail Service was a significant mail transportation service in the US from the mid-19th century until the mid-20th century. The ...
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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." - (en.wikipedia.org 01.03.2020)