Armstead is a former community in Beaverhead County, Montana, United States, about 21 miles south of the county seat of Dillon. It was named after miner Harry Armstead. Armstead was located in a narrow valley of the Beaverhead River, near the mouth of Horse Prairie Creek. The town was a station stop on the Union Pacific Railroad line from Idaho Falls…Armstead is a former community in Beaverhead County, Montana, United States, about 21 miles south of the county seat of Dillon. It was named after miner Harry Armstead. Armstead was located in a narrow valley of the Beaverhead River, near the mouth of Horse Prairie Creek. The town was a station stop on the Union Pacific Railroad line from Idaho Falls, Idaho to Butte, Montana. There was a post office in Armstead from 1907 to 1962.