In the 4.75 mile long Hoosac Tunnel in Massachusetts, growing traffic volumes at the beginning of the century caused major problems for the operation with steam locomotives due to smoke and steam. So the Boston & Maine planned to use electric locomotives to pull trains with their steam locomotive through the tunnel. With the steam locomotive idling, it created much less smoke and steam. So they electrified the tunnel with 11.000 V AC at 25 Hz in 1910.

Five electric locomotives were ordered from Baldwin and Westinghouse and numbered 2501 to 2505. They were of conventional boxcab design and rode on two bogies consisting of two powered axles and a leading axle. Three got a gear ratio for freight service, while the two others got a faster ratio for passenger service. Just one year later they were renumbered 5000 to 5004. In 1917, two more were built which were the first to be retired in 1942 and 1945. After diesel locomotives took over, all were retired until 1947.

After the electrification of the mountain lines over the Rocky Mountains, the Milwaukee Road also took on the Coast Division in Washington. This required an electric locomotive that could haul heavy passenger trains faster than the steam locomotives available at the time without double-heading. While the Rocky Mountains had many long, straight stretches on the plateaus, here the inclines of the Cascade Mountains had to be negotiated. A total of five of the EP-2 were built in 1919, costing $200,000 each and nicknamed “Bipolar” because of the two-pole traction motors used. They were built parallel to Baldwin-Westinghouse's EP-3 because the US Railroad Administration dictated orders to be split between manufacturers.

A total of twelve powered axles were required to achieve the required power. In order to achieve sufficient running characteristics in curves, the chassis was divided into four groups that were coupled to each other. The two outer groups each comprised a leading and two powered axles, while the two inner groups consisted of four powered axles each. The body consisted of three parts and each was articulated between two chassis groups. The electrical equipment and a cab were located in the outer parts, while a boiler for the train heating was housed in the middle part.

Although only five were built, they were the flagship of the Milwaukee Road, hauling important long-distance trains like the Olympian. Despite being designed for 90 mph, they were only approved for 70 mph, which was sufficient in their area of application. In 1939, the locomotives that previously had the numbers 10250 to 10254 were renumbered E-1 to E-5.

In 1953 the heavily worn locomotives were completely rebuilt. In addition to a modernized electrical system that allowed higher speeds and multiple controls, they were also streamlined. Shortly thereafter, the locomotives were transferred to the Mountain Division. It was found that the last rebuild, which had been carried out by a team not specialized in electric locomotives, had not been carried out optimally. As damage and failures now occurred more frequently, all locomotives were retired by 1960.

After the Coast Division was electrified via the Cascades, the Milwaukee Road procured very large passenger locomotives for the increasingly heavy trains. Due to the allocation of orders between manufacturers made by the US Railroad Administration, a further ten EP-3 from Baldwin-Westinghouse were built in parallel to the five EP-2 from General Electric. Although the EP-3 was heavier, it appeared more graceful in general.

The locomotive body was not split here, which made greater demands on the layout of the chassis. In principle, the locomotives each consisted of two chassis groups, each of which resembled a Pacific steam locomotive and were coupled to one another. Each axle was driven by a double motor, which was mounted in the frame and was therefore not part of the unsprung mass. Power was transmitted via a quill drive, which gave the locomotives the nickname “Quills”. Above the coupling between the chassis parts was the oil-fired oven for the train heating,

Due to the relatively rigid overall design and the lack of flexibility of the axles, the great wear and tear on the wheel flanges soon became apparent during operation, and there were also derailments. In retrospect, the weight-saving design of the locomotives did not prove to be advantageous either, so that the frames broke. Splitting the locomotive body of one of the locomotives into two parts did not help and even several modifications to the chassis and other parts of the locomotives could not eliminate all problems.

After the Second World War, the EP-4 „Little Joe” took over the duties of the EP-3. While other electric locomotives, badly worn out during the war, had been rebuilt, this was not considered useful for the EP-3. Thus, the seven remaining vehicles were all scrapped between 1952 and 1957.

After the Milwaukee Road had electrified 438 miles in Montana with 3,000 Volts DC, they also ordered four electric switchers from General Electric. These locomotives were built in 1916 and 1919 and were designated class ES-2. They were steeplecab locomotives with a B-B wheel arrangement and a central pantograph on the cab roof. To raise the pantograph with pressurized air when the battery was empty, an emergency trolley was provided to power the compressor.

The locomotives were only used in Montana in the Butte area, while the Milwaukee Road's other electrified section in Washington preferred steam and later diesel power for switching. With an output of 670 hp over one hour and 475 hp continuously, they later got additional weights to improve adhesion. One was scrapped in 1952 because it was surplus, but the others survived until the end of electric traction on the Milwaukee Road in 1974.

In the twenties, the Great northern built the new Cascade Tunnel and electrified the complete 73 mile (117 km) section over Stevens Pass with 11,000 V one-phase AC. Previously, only the old tunnel itself was electrified at 6,600 V three-phase AC and they now needed a replacement for the old GE boxcab electrics. So they ordered six 1-C+C-1 boxcabs from ALCO, which got their electric part from General Electric. They were numbered 5010 to 5017 and became class Y-1.

The alternate current from the overhead line was changed to direct current by a motor-generator. Over one hour, the locomotives delivered 3,300 hp, with a continuous output of 3,000 hp. In World War II, 5011 was badly damaged in an accident and its remains were used for a total rebuild where it got the streamlined cabs from the EMD FT.

In 1956, the Cascade Tunnel was dieselized and equipped with large fans to carry away the exhaust fumes. Since the frame of 5011 had been bent in the accident, it had several problems in service and was scrapped. All catenary was removed and the remaining locomotives were sold to the PRR, which called them FF1. The PRR scrapped three of them in the following years, but the last four were used until 1966.