A Union Pacific EMD GP40X with HT-B trucks.

The Electro Motive Division GP40X is a type of four-axle, 3,500hp diesel locomotive built from 1977 to 1978.

It is a rare variant of the EMD GP40, with only 23 built and only half still remaining on various leasing, shortline, and industrial companies.


The EMD GP40X was an experimental type of four-axle diesel locomotive, which was part of EMD's development of their prime-movers to get more horsepower and newer technology into their range. It foretold of the GP50, SD50, GP60, and SD60, such as introducing microprocessor controls, digital speedometers, automatic fuel starters, as well as having a form of distributed power known as "locotrol," which involved the ability for multiple locomotives to be controlled at once from the lead unit via a remote-control radio system without the need of crew members activating or starting other units' motors or engines manually. Optional HT-B trucks were also produced for improved dynamic braking and suspension, yet they actually caused issues as opposed to improving them.

The EMD GP40X was an initial success, and led to the success and introduction of the GP50. 


The EMD GP40X should not be confused with its original counter-part the GP40.

The main way of distinguishing it from other EMD four-axle diesel locomotives, is that it has flared radiators like the six-axle SD45 — however, the SD45 has three radiator grilles. Also some units, such as UP's, have HT-B trucks.


  • The Southern Pacific, Union Pacific, ATSF, and Southern Railroad (US) were the only railroads to purchase the GP40X-- ATSF had 10, UP had 6, SOU had 3, and the SP had two two-unit orders.
  • EMD #433A (the locomotive didn't have an actual type name) was a 1960s experimental GP40 predecessor sometimes called a GP40X, unrelated to the later GP40X. It was eventually purchased by the Illinois Central and rebuilt into what was known as a GP45 or GP50X, but was just a GP40 with flared radiators. (The unit was similar to the SD44R and shared a similar fate.)
  • "Elephant ears" were fitted to SP GP40X units as an experiment to draw cooler air into the radiators, similar to the earlier experiments with some SD45s and the SD40T-2 and SD45T-2s. They were eventually removed.
  • The HT-B truck was available on the GP50 during production, but was never ordered.
  • The TPW shortline owned by RailAmerica (now G&W) owns a former SP GP40X which still has its original trucks.
  • The ATSF's GP40X units never had the optional HT-B trucks.
  • Like every other EMD diesel locomotive purchased by the Southern Railroad other than the GP59, the GP40X had a "hi-hood" variant built specially for the railroad.


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