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 EMD SD40, but built on the shorter EMD SD35 frame. This locomotive was also known as an SD35X. The designation SD40X was also given to an experimental version of the EMD SD50, also built by EMD on an SD40-2 frame. 4 examples of this SD40X were built in 1979 and were delivered to the KCS as KCS 700-703.