For much of the history of Chicago commuter rail, a locomotive working one route could easily resemble one working another. The quintessential EMD F40PHs eventually became nearly universal across the Regional Transportation Authority and subsequent Metra systems, appearing first on the Rock Island, Burlington Northern. The Milwaukee District, however, has twice been home to something remarkably different.
Beginning in 1974, fifteen massive six-axle F40C locomotives entered service on the Milwaukee Road's Chicago commuter lines. Although they were numbered for the Milwaukee Road, they were purchased by suburban public transportation districts that were created to preserve and improve commuter service. These fifteen F40Cs would be the only ones of their kind ever built, and all spent their entire working lives hauling commmuters over the Milwaukee Road and, later, Metra.
Nearly half a century later, six-axle locomotives returned to the same territory. Beginning in 2022, Metra began receiving SD70MACH locomotives rebuilt from former freight SD70MACs. Large, heavy, and visually unlike any other locomotive in Metra's fleet, these locomotives have spent their working lives on the lines north of Chicago Union Station: the Milwaukee District North and West lines, as well as the ocassional trip on the North Central Service.
Once again, Chicago commuters are riding behind another unique commuter locomotive in the United States.