The next power level after the GP30 was the GP35, that was introduced in 1963. Thanks to improvements to the traction motors, 2,500 hp could now be obtained from the EMD 567D prime mover, which has been in use since the GP20 and has not yet been able to exploit its full power potential in four-axle locomotives. Production took place again in both the USA and Canada and a total of 1,334 pieces were made. Of these, 24 went to the Canadian Pacific, two to the Canadian National, 55 to the Mexican State Railway, two to the Sonora-Baja California Railway and the rest to US customers. The largest customers in the US were the Santa Fe, the Pennsylvania Railroad and Southern Pacific, which each ordered more than 100 examples. Many railroads got their GP35s while trading old F-series and Alco locos to EMD. There, the bogies were reused together with the traction motors and fitted with new superstructures.
Nine months after the GP35, the GP28 was added to the range. It was a cheaper variant that only had an installed power of 1,800 hp. The power came from the same engine, which only had a Roots supercharger instead of the turbocharger and was therefore in roughly the same power class as the GP9 engine. However, the GP9 was still using the old EMD 567C, while the GP28 had the more modern EMD 567D1. However, sales were limited and only 31 were built, 15 of them for Mexico and Peru. Other very similar locomotives were created when the Burlington Northern bought some GP9s from the Northern Pacific and brought them up to the same technical level. These locomotives are referred to as GP28M.
