The DC-motored SD70M-2 and AC-motored SD70ACe are EMD's hope for the future. While designed to meet the Environmental Protection Agency's Tier-2 emissions requirements that took effect on January 1, 2005, they also have a higher purpose: to recapture the lead in North American locomotive sales that EMD lost to General Electric in 1987.
Under the hood of both engines beats a third-generation model 710 diesel with 4300 horsepower; only slight modifications were needed to make the model 710 meet new emission standards. With 5000 such motors in service worldwide and a reputation for dependability, EMD reasoned that shop crews would prefer familiar technology.
Other than the prime mover, however, virtually every element of these engines has been re-thought to create a 21st century locomotive. Ergonomics were a prime consideration. The engines' angular nose offers the crew far better visibility than most other locomotives, and the cab is comfortable for engineers of almost any size. Digital screens provide a range of information on what is happening both inside the locomotive and out on the road. The cab easily accommodates a crew of three - an important factor in a modern world without cabooses. And there is, of course, a cupholder for the engineer.
The key difference between the SD70M-2 and the SD70ACe is what's under the floor: traditional DC traction motors in the SD70M-2's trucks and AC traction motors in the SD70ACe. While AC traction motors put more tractive effort on the rails and enable an SD70ACe to start a heavier train with the same horsepower, that additional capability comes at a cost. AC-powered locomotives are both more expensive and more electrically complex than engines with DC traction motors, which most diesels have used since the 1940s. For that reason, the Norfolk Southern and Canadian National initially found the SD70M-2 more suitable to their needs, and both roads roster large DC fleets. Regional railroads Florida East Coast and Providence & Worcester have also been DC-power advocates. In the last several years, however, even those railroads have moved to AC power for new purchases, and the last order for a new SD70M-2 was in 2010.
