Another of the many impressive engineering feats accomplished during the construction of the Milwaukee Road's Pacific Coast Extension was its crossing of the Continental Divide at Pipestone Pass, which was along the eastern edge of the Rocky's foothills in a range known as the Pioneer Mountains. Interestingly, the pass was not far from the Jefferson River Valley and to keep grades reasonable the railroad's engineers built the equally impressive Vedome Loop, which was only a few miles to the east. Today, like the loop Pipestone Pass Tunnel remains quiet as trains have not traversed the route in more than 30 years.
Few rail lines, let alone railroads, have garnered such interest, intrigue, and awe as the Chicago, Milwaukee, St. Paul & Pacific Railroad's Pacific Coast Extension, even 30 years since it was abandoned. Unfortunately, the best engineered rail line through the rugged Cascades could not save it from the inept decision making of management at the time and it is now but weeds and trails, a vital transportation artery no longer available to shippers and the American economy. In 1901 the first surveying work began and it was estimated the more than 1,400-mile western extension would cost the railroad around $45 million adding more than 25% to its total system mileage.
However, four years later this number was readjusted to $60 million. What made the extension so terribly expensive was partly due to the right-of-way costs. Unlike the GN and NP the CM&StP was not given free government land grants and had to both purchase all of its land from private landowners as well as take over a number of small, new, or floundering railroads across the region. Amazingly, in just three short years the entire extension had been completed and on May 19, 1909 a Golden Spike was driven at Garrison, Montana commemorating the opening of the new route.
The Pipestone Pass Tunnel was part of one of many segments engineers and planners split the Pacific Coast Extension into during surveying so as to construct the route as fast as possible (all over the states of Montana, Idaho, and Washington parts of the line were being built simultaneously). The pass and Continental Divide were less than ten miles south of Butte, Montana and along with the route's close proximity to the Northern Pacific Railway's main line helped immensely in moving men and material to the site quite quickly and easily. Survey work on this section of line between Lombard and Butte began in 1904 with surveyors determining that the lowest grade route over the Pioneer Mountains was through Pipestone Pass at the Continental Divide.
Construction of the tunnel and right-of-way was carried out by the McIntosh Bros. and by 1908 a temporary line had been completed between Three Forks and Butte although the tunnel itself was yet to be completed making for a tortuous climb over Pipestone for about a year. However, the work was not easy. Loose and solid granite rock along the way made grading very difficult resulting in the route's opening being slightly delayed and costing more than anticipated. The tunnel, officially known as #11 on the Milwaukee Road and located at Milepost 1505.5, itself was completed and opened in 1909 (general boring and drilling had been completed during 1907 and 1908).
By the early 20th century most railroad tunneling projects were being constructed entirely of concrete to reduce maintenance costs and increase strength and Pipestone Pass Tunnel was no different (actually, all of the tunnels along the Pacific Extension were built this way). The tunnel sat an elevation of 6,347 feet and was about a half-mile in length at 2,290 feet. Overall, the route between Butte and Three Forks through the mountains ranged in grade from 1.66% to 2%, which was quite good considering roughly ten miles east of the tunnel the mountains drop away to Jefferson River Valley. To keep the grade manageable engineers and surveyors designed the impressive 3-mile Vendome Loop.