In the late 1800's the Milwaukee Road was a prosperous railroad out of Chicago that had over 6,000 miles of track in the upper Midwest. Experiencing competition from other rail lines, the company decided to expand west to take advantage of the expanding West Coast markets, as well as the Pacific Rim trade.
The route proposed for the new line was through the rugged Bitterroot Mountains. Prior to preparing official plans for the construction, significant exploration had to be undertaken to select the most feasible route. Various possibilities existed westward of Butte and up beyond Missoula. The exploration and reconnaissance crews reportedly covered over 2,000 miles in country that was wild, uninhabited, with very few trails, and virtually no maps.
The exploration work on the Montana side began in November, 1904. The exploration on the Idaho side began in May, 1905. In November of 1905, the Milwaukee Railroad Board of Directors formally approved the lines extension to Seattle-Tacoma, Washington.