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The Pittsburgh and Shawmut Railroad hauled coal over eighty-eight miles from Brockway in Jefferson County to Freeport in Armstrong County. In southwestern Jefferson County, a pair of 142-foot-high viaducts between the small towns of Coolspring, Markton, and Sprankle Mills are joined by the Coulter Tunnel. Sprankle Mills' steel viaduct is 1,430 feet long and is best seen from PA 3014.