An EMD GP40X is a 4-axle diesel-electric locomotive built by General Motors Electro-Motive Division between December 1977 and June 1978. Power for this unit was provided by a turbocharged 16-cylinderEMD 645F (derivative of the EMD 645 series engine) which could produce 3,500 horsepower (2,610 kW). 23 examples of this locomotive were built for North American railroads. This unit was a pre-production version meant to test technologies later incorporated into EMD's 50-series locomotives GP50 and SD50. Ten GP40X were delivered with an experimental HT-B truck design that became an option (but never used) on the production GP50.
The designation GP40X was also given to an experimental locomotive built on an EMD GP35 frame in May 1965. Only one example of this locomotive was ever produced, the EMD 433A, a 3,000 hp (2,240 kW) test prototype that was the first 4-axle locomotive to be powered by the new 645-series prime mover. The 433A served as the precursor to the EMD GP40. The 433A was purchased by the Illinois Central Railroad, and became the IC 3075. The 1965 EMD 433A has very little in common with the 1977 GP40X other than flared radiators and a 645-series prime mover.