The GP50/SD50 series built from 1980 introduced some technical features which had been tested in the GP40X prototypes. The new HT-B trucks were never used in production, but other features taken over from the GP40X were the new radiator coverings which were called “elephant ears”, and the new “Super Series Wheel Slip Control” that uses a Doppler radar to monitor wheel slip and reduce power when needed. Additionally, they introduced microprocessors into production locomotives.
The GP50 was equipped with an optimized 16-645F3B prime mover that delivered 3,500 or 3,600 hp. With still only 16 cylinders compared to the SD45's 20, the frames could remain as short as in the GP40. New traction motors were introduced which could cope with the increased power being split among only four axles. In total 278 GP50 were produced for five customers, most of which have either been scrapped or rebuilt in the meantime.
The GP49 was a variant with a reduced power of 2,800 hp introduced in 1983. In this case, Alaska Railroad was the only customer with four locomotives built in 1983 and five in 1985. Six GP39X built for the Southern Railway were later rebuilt to GP50. In 2006 Alaska Railroad started to sell their GP50 and two of them went to Ferronor in Chile where they were rebuilt to six axles. The six rebuilt by the Southern Railway were last used by commuter operator Tri-Rail in Florida. There they had been equipped with head-end power.