Taken from the wikipedia website. The Burlington Northern Railroad was the most extensive rebuilder of GP30s. Finding a need for modernised units of lower power, it sent GP30s—both its own and purchased from others—to be rebuilt. Some (70 units) were sent to EMD to be rebuilt with EMD 645-series engines, new generators, and Dash-2 modular electronic control systems; they are rated at 2,300 hp (1,720 kW), and are designated GP39E. Others (65 units) were sent to Morrison Knudsen (now MotivePower Industries) for similar work; these are rated at 3,000 hp (2,250 kW) and are designated GP39M. These units are still in service on local and smaller lines throughout the BNSF Railway system.
The Atchison, Topeka and Santa Fe Railway, meanwhile, performed a similar upgrade in its own Cleburne, Texas shops, stripping the locomotives down to bare metal and rebuilding with new equipment. EMD 645 engines rated at 2,500 hp (1,900 kW) replaced the old; the generators and traction motors were also upgraded; control and electrical equipment was replaced. The trucks were upgraded with Hyatt roller bearings and new brake systems. Rooftop air conditioners were added, and new horns. The locomotives were repainted in the blue and yellow Yellowbonnet scheme, and designated GP30u (for upgraded). 78 of these survived until the BNSF merger, and most are still in use on secondary service.