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 crossing the territory of many Tribal nations along the way in country that was wild, 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.

Because time was an enormous factor, the crews worked year round and did not stop because of winter weather. By the spring of 1906 the choices were narrowing to the St. Paul Pass and St. Joe River valley. After the exploration surveys, during most of 1906 engineering survey crews finally helped select and identify the actual location of where the tracks would be located.

By early in 1907 the construction work began. The actual construction of the rail bed and the track was very difficult due to the forbidding terrain and the weather conditions. The cost of the project which was originally estimated at $45 million, ended up exceeding $234 million, (plus another $23 million to convert to electric locomotives in 1910 to 1911). All in all it took nearly 9,000 men, Italians, Serbs, Montenegrins, Austrians, Belgians, Hungarians, Japanese, French, Canadians, Spaniards, Irishmen, Swedes, Norwegians, and others all working together from 1906 to 1911 to construct this Pacific extension.

Intercontinental freight service on the new line began on July 4, 1909, with passenger service following six days thereafter.

As the construction proceeded, numerous settlements sprouted throughout the area. Avery was the division point on the line and became one of the more substantial settlements. Others were just very rough construction camps, with evenings full of riotous gambling, drinking, dancing, and fighting.

In August of 1910 one of the most devastating forest fires in recorded American history burned much of the natural forests in Northern Idaho and Western Montana. The fire burned 2½ to 3 million acres. It was so huge that a massive cloud of smoke spread throughout Southern Canada and the Northern United States all the way to the St. Lawrence waterway. The darkness from this smoke was so bad that for 5 days artificial lighting had to be used from Butte, Montana including Chicago to Watertown, New York. The fire completely devastated the St. Joe River valley and destroyed all of the towns except Avery and Marble Creek. Many of these were never rebuilt.

There were numerous stories of very heroic actions by the railroad employees who drove engines and box cars filled with people through the flames to the safety of the longer tunnels. Reportedly over 600 lives were saved in this manner alone.

After this disastrous fire, as well as for some other operating reasons, the Milwaukee Road made the decision to electrify the line (use electric locomotives) between Avery and Harlowton, Montana, a distance of 440 miles. This innovation by the railroad was the first use of electrification over an extended distance and was watched over very closely by other railroaders. The results were astounding both in terms of reliability of operations as well as profitability. These were the glory days of the railroad, which was then called the Chicago, Milwaukee and Puget Sound (later to be the Chicago, Milwaukee, St. Paul and Pacific).

Still, despite its accomplishments, the railroad had more than its share of hardships. It was forced into receivership in 1925. It was reorganized 18 months later. In the years between 1921 and 1940 it only had three years in the black. The railroad declared another bankruptcy in 1935. The advent of the speedy 100 m.p.h. Hiawatha train in the Midwest, and the Olympian Hiawatha from Chicago to Seattle, returned some degree of prosperity to the line following World War II. But even that was short-lived, and gradually through various reductions of services this once proud railroad gradually deteriorated. The last passenger train, the Olympian Hiawatha passed through the Bitterroots in 1961, and the electric locomotives where gradually replaced by diesel engines by 1973. The final bankruptcy was filed in 1977, and the last train west of Butte passed through in 1980. After that the line was abandoned.