The Chicago, Milwaukee, St. Paul and Pacific Railroad Company Historic District consists of the historic right-of-way of the Chicago, Milwaukee, St. Paul and Pacific Railroad (also known as The Milwaukee Road) in the Bitterroot Mountains from East Portal, Montana (near St. Regis), to the mouth of Loop Creek, Idaho (near Pearson), a distance of 14.5 miles (23.3 km).

This pass was chosen because of the stands of marketable white pine timber and also because there was no other competing railroad nearby.

The construction of this segment of the Pacific Extension was the most expensive, due to the remote location and rugged terrain.

In addition, the district may be able to help archaeologists learn more about the early construction of the railroad and the crews that did the work.

For these reasons, the 14.5 miles (23.3 km) of the Milwaukee Road's Pacific Extension through the Bitterroot Mountains were listed in the National Register of Historic Places.