E57B, built by General Electric in 1915-1916, was one of 42 of the EF-1 class (84 half locomotives semi-permanently connected together) which operated on the electrified section of the Chicago, Milwaukee, St. Paul and Pacific Railroad, more commonly known as the Milwaukee Road. The section was 656 miles in length, stretching from Harlowton, MT to Avery, ID and from Othello, WA to Tacoma. WA in two not adjoining sections.
The Milwaukee turned to electrically driven locomotives to pull trains over the mountain passes between Harlowton and Tacoma, WA. It was seen that electric locomotives were capable of greater pulling power tham steam locomotives, while requiring less maintenance and incurring lower "fuel" costs. In all, there were 116 electric locomotives built to service the electrified sections of the Milwaukee Road's western division which passed over five mountain ranges. Weighing 564,000 pounds, each unit produced up to 4,100 horsepower (for 1 Hour - 3,340 HP in continuous operation) using 3000 Volts of direct current electricity. The advent of diesel locomotives proved to be the death knell for the electrics, though they managed to continue in operation for a couple of decades following the introduction of the diesels.
Technical Information
They were fitted with multiple-unit train control systems, and could thus be joined together into larger sets and operated from a single control station. They were also retrofitted with a special multiple unit control system designed by an electrical engineer of The Milwaukee Road. This enabled the crew of a boxcab to control trailing diesel electric locomotives. However, the EF-4 "Little Joes", which were also retrofitted, were more often seen leading diesel electrics than boxcabs, which had by then been largely relegated to the role of helper or bank engine.
The maximum speed of an EF-1 as built was 35 miles per hour (56 km/h). Higher speeds led to excessive strain on the traction motor armatures. The rebuilding program of the 1950s raised this to 45 miles per hour (72 km/h) to help maintain faster schedules.
The two powered trucks were connected together with a ball-and-socket joint, and the couplers were also attached to the trucks. The bodywork, therefore, did not take the load of the train. Each truck had outside bar frames, allowing more room for the traction motors and equipment. The front powered truck's frames extended forward and carried an outrigger truck and the heavy snowplows the units bore.
From Wiki
Following is text from the historical marker at the locomotive.
E57B The Last Electric Locomotive
The Milwaukee Road: 656 mile electrified railroad ended at 11:40 PM June 15, 1974 when Engineer Art Morang stopped the E57B and E34C on the Harlowton roundhouse track. They were the last operating locomotives of the original 84 locomotives built by General Electric in 1915.
The electric locomotive roster had totaled 116 locomotives of 5 different types operating from Harlowton, MT 440 miles to Avery, Idaho and 216 miles from Othello Wash. to Tacoma Wash. over 5 mountain ranges.
The E57B is 57'8¾" long, 16'8" high (paragraph down), 10'0" wide and weighs 144 tons. Rated at 1500 h.p., it could develop 2395 h.p. starting effort at 62% overload. Operated in 1 to 4 unit consists, they were very trouble free locomotives. The 3000 volt D.C. trolley restricted them to a small portion of the 11,248 mile railroad and they were replaced by the more versatile diesel electric locomotives.