The Pacific Extension was the successful attempt by the Chicago, Milwaukee and Saint Paul Railroad (the Milwaukee Road) to build a transcontinental line over the Rocky and Cascade mountain ranges and reach the Pacific Northwest, specifically the Seattle area. Construction on the line began in 1906 and completed in 1909; the line represented the shortest route over the Continental Divide. Upon completion, the name of the CM&StP was changed to the Chicago, Milwaukee, Saint Paul and Pacific Railroad, adding "Pacific" to the name in order to identify its extents. This route was the last of the great transcontinental routes to be constructed in the United States.
Due to the northern regions through which the line traveled, the Milwaukee Road found that electrifying various parts of the Pacific Extension produced more favorable results than the usual steam power that was typically found elsewhere during that time. At the height of electrification, 656 total miles along the extension were electrified (albeit in different locales and therefore disconnected), making the Milwaukee Road the largest electrified railroad in the US. It dismantled its electrified infrastructure in 1973, during the onset of the oil crisis -- as diesels started roaming in previously electrified territory and with oil prices soaring, the de-electrification of the Milwaukee Road ultimatley proved ill-timed.
Because of the Milwaukee Road's increasing financial woes during the 1970s, the track along the Pacific Extension slowly deteriorated due to lack of funds for maintenance. The road's bankruptcy in 1977 ultimately spelled doom for the Pacific Extension, and it was abandoned west of Miles City, MT, in 1980.
Thanks to Mike Fromholt for contributing information about this route.