EMD GP40X

The EMD GP40X was a small production run of pre-production diesel-electric locomotives constructed by General Motors' Electro-Motive Division between December 1977 and June 1978. A total of 23 units were built during this period and distributed among a limited number of North American railroads, with Southern Pacific Transportation Company being among the principal recipients. Rather than representing a conventional production locomotive intended for widespread sale, the GP40X was conceived as a testbed for technologies and engineering concepts that EMD intended to evaluate before committing them to full-scale production. The significance of the GP40X lies primarily in its role as the developmental predecessor to EMD's 50-series locomotives, most notably the GP50 and SD50. The lessons learned from operating the GP40X in revenue service allowed EMD's engineers to assess new truck designs, adhesion systems, and prime mover configurations under real working conditions. This approach of fielding a small pre-production series before launching a major new locomotive family was consistent with EMD's effort to maintain its competitive standing during a period when rivals were also advancing their own high-horsepower road locomotive designs. The designation GP40X had actually been applied once before, to a single experimental locomotive constructed in May 1965 on a GP35 frame. That earlier unit, known internally as EMD 433A, was a 3,000 horsepower prototype and the first four-axle EMD locomotive to use the then-new 645-series prime mover. It was subsequently purchased by Illinois Central Railroad. Despite sharing the designation and certain superficial characteristics with the later 1977 production run, the two groups of locomotives had very little else in common technically or historically.

The GP40X rode on a B-B wheel arrangement and was powered by a turbocharged 16-cylinder 645F3B prime mover producing 3,500 horsepower, making it a meaningful step up in output from the standard GP40. The locomotive used direct current traction, consistent with EMD's prevailing electrical systems of the era. One of the more consequential engineering features evaluated on the GP40X was an experimental HT-B truck design fitted to ten of the units, including six delivered to Union Pacific and four to Southern Pacific. These trucks were intended to improve tractive effort and adhesion compared to the conventional Blomberg trucks fitted to the remaining units in the series. The HT-B design was subsequently offered as an option on the production GP50, though no railroad ultimately ordered it in that configuration. Southern Pacific's four GP40X units were notable for incorporating several additional experimental modifications beyond the truck design. These included L-shaped windshields and distinctive coverings over the radiator sections that earned the informal nickname "elephant ears" among railroad observers and crews. These features reflected Southern Pacific's willingness to collaborate with EMD on testing unconventional design elements, and they gave the SP examples a visually distinctive appearance that set them apart from other members of the class.