The EMD SD50 is a 3,500-horsepower  locomotive built by General Motors Electro-Motive Division. It was introduced in May 1981 as part of EMD's "50 Series"; production ceased in January 1986. The locomotive was released by the company in the early 1980s and was another attempt to provided a higher-horsepower model to keep pace with General Electric's increasing competition and its C36-7 model that offered more power than the SD40-2.

Unfortunately, EMD rushed the SD50 into production and it not only suffered mechanical problems but also did irreparable damage to the company's reputation as a respectable locomotive builder.  Interestingly, after the issues with the SD50 were corrected the locomotive did prove quite reliable. As a result, many many remain in use today.

The Rio Grande tested the SD50 borrowing four Seaboard System SD50's (8502/8507/8510/8521) which arrived on March 24th 1984 at Pueblo from the MP for a one-week test period. The big gray units were primarily between Pueblo and Craig on coal trains, but they also handled some general freight. The railroad made it a point to use the engines as much as possible in order to obtain a complete analysis of their performance in the difficult mountain terrain. They usually ran in and out of Denver at night but occasionally made daylight appearances on the road, much to the delight of lineside photographers. The cabs always seemed to be well-filled with officials who were compiling test data. Although the Rio Grande has signed a letter of intent with EMD for the purchase of 20 SD40T-2's in the fourth quarter of 1984, it would still be possible for the railroad to switch its order to SD50's instead. Initial reaction to the SD50's was very favorable. The four units were turned over to the Union Pacific at Denver on April 1, 1984 where they will be tested by UP personnel.