In 1905, the Milwaukee and St. Paul Railway sought a route for its western extension over the Bitterroot Mountains. After five and a half months and exploration covering 930 miles, the railroad selected a route over St. Paul Pass. From the St. Paul Pass Tunnel, surveyors planned a line descending at a 1.7 percent gradient along the mountain slope. A major factor in selecting this route was its potential for future traffic, since the route down the St. Joe River offered exclusive access to huge quantities of old growth white pine and cedar timber.