H.K. Porter Company in Pittsburgh, PA, built this diminutive 18″ gauge 0-4-OCA (Compressed Air) engine in 1910 for the Homestake Mining Company in Lead, SD. It has 6″ x 10″ cylinders. Compressed air was a suitable form of power for mining because it produced no noxious fumes or damp-causing steam. In 1901 the mine started using compressed air locomotives, replacing mules and horses by the 1920s.