Soon after the Milwaukee went way over budget finishing its Pacific Extension in 1908, it decided to spend even more money electrifying the portions of the route in the Rocky and Cascade mountains. Electrification was all the rage in the early 1900s; Pennsylvania, of course, electrified its lines from New York City to Harrisburg and Washington, DC, while Southern Pacific considered electrifying Donner’s Pass. When Milwaukee opened its electrified route in 1915, it was proud to say it had the longest electric line in the country.
Electrification of portions of the route made a lot of sense in the steam era when trains changed locomotives every few hundred miles anyway. But Diesels could go all the way to Seattle without being changed, so having to change them four times to go over two mountain ranges was a hassle. By 1973, the railroad was still using some of the original, 1915 locomotives, and its electric infrastructure was worn out. Even though General Electric offered to finance replacement costs, the company decided to go to an all-Diesel system.
It probably regretted that decision, as oil prices suddenly skyrocketed at about that time, which made the nearly free hydroelectric power seem pretty attractive. No doubt many Milwaukee fans wonder if the Pacific extension would have survived, instead of being scrapped in 1980, if the railroad had retained the electric system.