Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. An EMD SD40X is an 6-axle diesel locomotive built by General Motors Electro-Motive Division between 1964 and 1965. 9 examples of this locomotive were built for test purposes to test the new 645-series prime mover. After testing, eight were sold to the Union Pacific Railroad and one example to the Gulf, Mobile and Ohio Railroad. This locomotive is an experimental testbed for the production SD40, but built on the shorter SD35 frame. This locomotive was also known as an SD35X.