The UP 3399 was rebuilt from an SD24 as a prototype for a possible rebuild program to make them similar to the SD40s. The rebuild effort included a constant speed EMD 16-645 3000 horsepower engine... (more)

The UP 3999 was rebuilt from an SD24 as a prototype for a possible rebuild program by the Union Pacific, making them similar to the SD40s. The rebuild effort included a constant speed EMD 16-645 3... (more)