 This was the first model in EMD's SD "(Special Duty)" series of locomotives, a lengthened B-B GP7 with a C-C truck arrangement.The two extra axles and traction motors are useful in heavy, low speed freight service.SD series locomotives are still being produced today, with the SD70 being the most popular example in current production, and with many SD40-2s and rebuilds to SD40-2 specifications, or SD60s still in operation.