Interesting that the RPO is from Train 18 in 1907, as that same year, that train service was extended from May to September to year-round service (except Sundays) to New York City as referred to in this excerpt from the Postal Bulletin:
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Ultimately, the RPO, as well as the train service via the Catskill Mountain Branch of the New York Central Railroad, formerly the Ulster and Delaware Railroad, ended in 1954 as per this news article thanks to the demise of the railroads in general in deference to truck and automobile transportation being made to the area:
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