Between 1914 and 1916, the Milwaukee Road undertook the largest mainline electrification to date by equipping their 438-mile Rocky Mountain line with 3,000 V DC. This included longer grades with two percent and so, powerful electric locomotives had to be built for both freight and passenger traffic. ALCO and General Electric designed these boxcab locomotives, which were made up of two sections, each one having two trucks with two powered axles each and a two-axle leading truck.
A promotional film made by GE called these locomotives “Kings of the Rails”. 30 EF-1 were built for freight traffic which were geared for 35 mph, had an hourly output of 4,100 hp and a continuous output of 3,340 hp. Additionally, twelve EP-1 were built for passenger traffic which were geared for a higher top speed and had a steam heating boiler. Only a few years later, all EP-1 were rebuilt to EF-1 by re-gearing them to 35 mph and removing the boiler.
Later, new classes were created by combining more sections. This process started in the thirties when 18 EF-1 were combined to twelve three-section EF-2. Even though more EF-2 followed later, the EF-3 was also created from three sections. Here the unused cab of the central section was removed, alongside with the leading bogie. They still had roughly the same adhesive weight as the EF-2, but weighed 51,000 pounds less in total. Between 1951 and 1961, a total of ten locomotives was even created with four sections and called EF-5. A single section that had been left over was used as switcher and designated ES-3.
In 1950, two were rebuilt for passenger service. One of those received streamlined fronts and now had an output of 4,500 hp. Another rebuild was the installation of multiple controls used to control a trailing diesel-electric locomotive. The introduction of the EF-4/EP-4 “Little Joe” meant that this feature was only rarely used and the boxcabs were increasingly used as helpers. Nevertheless, the older locomotives were approved for 45 mph in the fifties. Their service ended in the mid-sixties and today, one two-section unit and a single section are on static display.