The Primrose Substation was one of thirteen Montana substations that stood sentinel next to the Milwaukee Road Railroad between Harlowton and St. Paul Pass on the Idaho border. Completed in 1909, the Milwaukee Road was the third and last transcontinental railroad to cross Montana. In 1916, the railroad electrified the line between Harlowton, and Avery, Idaho, to increase the efficiency of its operations. The Milwaukee’s 440-mile Rocky Mountain Division was the longest electrified railroad in the United States. Electrification required substations, like this one, spaced at thirty-seven-mile intervals along the railroad. High tension power lines fed 100,000 volt alternating current (AC) to the substations, where motor-generator sets, transformers, and switching equipment stepped down the current to 3,300 volts direct current (DC) for use by the railroad’s locomotives. Built in 1915, the Primrose Substation reflects the Milwaukee Road’s standard architectural design for this type of building. The two-and-one-half-story Industrial style building appears austere from a distance, but exhibits ornamental brickwork around the base and windows as well as decorative tiles along the cornice. The large south-facing, multi-light windows provided ample natural lighting for the building’s interior. The lightning arresters and horn gaps on the roof dissipated lightning strikes when they occurred. The Primrose Substation also functioned as a ticket office and waiting room for local passengers. The substation was an integral part of the Milwaukee’s Rocky Mountain Division for nearly sixty years until 1974 when the line converted to diesel power locomotives. The Milwaukee Road abandoned its line in Montana in 1980.