Prominent individual investors in the line included Alexander Mitchell, Russell Sage, Jeremiah Milbank, and William Rockefeller.

The company absorbed the Chicago and Pacific Railroad Company in 1879, the railroad that built the Bloomingdale Line (now The 606) and what became the Milwaukee District / West Line as part of the 36-mile Elgin Subdivision from Halsted Street in Chicago to the suburb of Elgin, Illinois.

The subsidiary Chicago, Milwaukee and Puget Sound Railway Company was chartered in 1905 to build from the Missouri River to Seattle and Tacoma.

The two main mountain ranges that had to be crossed, the Rockies and the Cascades, required major civil engineering works and additional locomotive power.

Original company maps denote five mountain crossings: Belts, Rockies, Bitterroots, Saddles, and Cascades.

Some historians question the choice of route, since it bypassed some population centers and passed through areas with limited local traffic potential.

[7] George H. Drury listed the Pacific Extension as one of several "wrong decisions" made by the Milwaukee Road's management which contributed to the company's eventual failure.

[8] Beginning in 1909, several smaller railroads were acquired and expanded to form branch lines along the Pacific Extension.

Electrification provided an answer, especially with abundant hydroelectric power in the mountains, and a ready source of copper in Anaconda, Montana.

[11] Between 1914 and 1916, the Milwaukee Road implemented a 3,000 volt direct current (DC) overhead system between Harlowton, Montana, and Avery, Idaho, a distance of 438 miles (705 km).

[14][15][16][17] Together, the 645 miles (1,038 km) of main-line electrification represented the largest such project in the world up to that time, and would not be exceeded in the US until the Pennsylvania Railroad's efforts in the 1930s.

[18] The two separate electrified districts were never unified, as the 216-mile (348 km) Idaho Division (Avery to Othello) was comparatively flat down the St. Joe River to St. Maries and through eastern Washington, and posed few challenges for steam operation.

[21] In 1927, the railroad launched its second edition of the Olympian as a premier luxury limited passenger train and opened its first railroad-owned tourist hotel, the Gallatin Gateway Inn in Montana, southwest of Bozeman, via a spur from Three Forks.

The premier transcontinental streamliner, the Olympian Hiawatha, despite innovative scenic observation cars, was mothballed in 1961, becoming the first visible casualty.

To boost competition, the ICC gave the Milwaukee Road the right to connect with new railroads in the West over Burlington Northern tracks.

[citation needed] In 1970, the president of Chicago and North Western offered to sell the railroad to the Milwaukee Road outright.

President William John Quinn refused,[28] stating that it now believed only a merger with a larger system, not a slightly smaller one, could save the railroad.

By the mid-1970s, deferred maintenance on Milwaukee Road's physical plant, which had been increasing throughout the 1960s as it attempted to improve its financial appearance for merger, was beginning to cause problems.

[29][30] In 1976, the Milwaukee Road exercised its right under the Burlington Northern merger to petition for inclusion based on its weak financial condition.

While the Burlington Northern merger generated more traffic on this route, it was only enough to wear out the deteriorating track, not enough to pay for rebuilding.

[37] A final attempt to devise a plan to rehabilitate the Pacific Extension under the Milwaukee Road Restructuring Act failed.

[39] Still in reorganization, the Milwaukee Road attracted interest from three potential buyers: the Grand Trunk Corporation, the Chicago and North Western Railway, and the Soo Line Railroad.

[40] The Soo Line would be acquired by Canadian Pacific in 1990 with the latter consolidating with the Kansas City Southern Railway 33 years later.

[42] Without the railroad, CMC's primary function became disposal or redevelopment of Milwaukee Road real estate not sold to the Soo Line, which stretched from Bedford, Indiana, to Washington state.

The CMC itself was beset with legal and financial woes, filing for bankruptcy (under numerous versions of CMC/Heartland Partners), as a result of environmental cleanup costs and liabilities at former Milwaukee Road sites.

Today, both the Milwaukee Road and Soo Line Railroad trackage make up the Midwest US routes of the CPKC.

After assuming operation of the UP's services, the Milwaukee Road gradually dropped its orange and maroon paint scheme in favor of UP's Armour yellow, grey, and red, finding the latter easier to keep clean.

Daily long distance service to and from the Pacific Northwest is provided by the Empire Builder along the Chicago-St. Paul route after the train was rerouted by Amtrak on the first day of operations on May 1, 1971.

In 2024, Amtrak began service for the Borealis, supplementing the Empire Builder with an extra daily round trip from Chicago to St. Paul.