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 3000 horsepower engine (with speed control taking place by changing the level of excitation of the main generator) and traction motors permanently connected in parallel. UP tried many other features, including a variable dynamic braking feature, first used on the small 51-75 single unit turbines. UP unit was also the first unit on UP to have the capability of a self-load test, rather than being connected to an external load box. This self load feature was first delivered on UP's DDA40Xs, and on almost all production units from EMD after 1971. The more troublesome test features were soon removed; the constant speed engine only lasted about two years, the traction motors were also connected in a normal fashion at that time. When built, the unit was equipped with fiberglass hood doors, with magnetic latches. The latches were changed to the EMD standard "knuckle buster" design in March 1974 when the unit received a new generator in Salt Lake City. Chronologically, the UP 3999 was built new by EMD as UP SD24 No. 423 in Sept. 1959, rebuilt the UP SD24M 3100 in August 1968, with new SD40-2 deliveries from EMD it was renumbered the UP 3200 in December 1970, the UP 3399 in March 1972, the UP 3999 in November 1976, and finally the UP 99 in October 1978. It would be retired and sold for scrap to St. Louis Auto Shredding, East St. Louis, IL on November 12, 1985. (information courtesy UtahRails.net)
