EMD introduced the GP15-1 in 1975 as a replacement for aging GP7s and GP9s still serving in switching, transfer and wayfreight service. Offering 1500 horsepower, the new design was better suited to the bigger and heavier cars then entering service, while the low nose improved forward visibility for added safety. As an added incentive, EMD required customers to provide a trade-in unit, which supplied traction motors, trucks and main generators, reducing the purchase price. Outwardly, GP15-1s looked like 35 Series engines, with a road-style cab and short nose. The most distinguishing feature was the single 48" fan and tunnel motor air intake for the radiators. Later versions included the GP15-1AC, equipped with an alternator instead of a generator and the GP15T, which added a turbocharger to the AC model. Nearly 370 of the three styles were built for US railroads between 1976 and 1983, and many are still going strong today.