Chicago, Milwaukee, St. Paul and Pacific Railroad Company Historic District, Railroad Historic District in Bitterroot Mountains, Idaho.

The Chicago, Milwaukee, St. Paul and Pacific Railroad Company Historic District follows 14.5 miles of the Milwaukee Road right-of-way through the Bitterroot Mountains, from East Portal, Montana, to the mouth of Loop Creek, Idaho. Within this stretch the railroad built 14 tunnels and 26 bridges; the tunnel at St. Paul Pass measures 8,771 feet. Bridges, tunnels and some catenary poles remain standing.

The Milwaukee Road began its Pacific Extension in 1906, surveying a route that crossed the Bitterroot Mountains at St. Paul Pass and followed the St. Joe River to Avery, Idaho. This segment cost the most of the extension because of its remote setting and rough terrain; crews bored 14 tunnels and built 26 bridges, including the 8,771-foot St. Paul Pass tunnel. The line was electrified from 1914, electric operation ended in 1974, and the railroad went bankrupt in 1980.

The route gained importance because of abundant white pine forests in the area and the absence of competing rail lines nearby. This advantage shaped how the railroad company chose its expansion path through the mountains.

The area is now accessible as the Route of the Hiawatha Trail for cycling and hiking, allowing visitors to pass through historic tunnels and across bridges. The best time to visit is during warmer months when the mountain passage is fully accessible.

The line operated with electric power from 1914 to 1974, with substations at East Portal and Avery supplying trains through overhead catenary lines. This advanced technology was uncommon for mountain rail routes in the region during that era.