MTH ELECTRIC TRAINS

Serious efforts to use AC (alternating current) diesel-electric engines began when General Motors introduced its 4,000-hp SD-60MAC in 1991-92. While the AC power increases an engine's adhesion, allowing it to start far heavier trains than a DC-powered locomotive of the same horsepower, the heavier load prevents AC-powered engines from reaching track speed. To solve that problem, locomotive builders have raced to design and deliver AC engines with ever-larger horsepower. In 1995 GM introduced the 5,000-hp SD-80MAC, which uses a 20-cylinder version of EMD's 710-series prime mover to generate extra power. While they were impressed with GM's results, most railroads chose to wait for the 6,000-hp SD-90MAC, but Conrail took the plunge and placed a 30-unit order for the SD-80MACs. The railroads that opted to wait for the 6,000-hp SD-90MACs initially found themselves with less power when their -90s were delivered in 1996. GM opted to deliver the engines with 4,300-hp prime movers that were designed to be replaced with 6,000-hp engines when they became available. Now that they are operating at full power with a GM16V265H engine, the SD-90MACs offer 170,000 pounds of continuous and 200,000 pounds of starting tractive effort, as well as 115,000 pounds of dynamic braking effort. M.T.H. offers the transitional SD-80MAC in the markings of the CSX and Norfolk Southern railroads, each of which inherited several from Conrail. A detailed version of Type 2 SD-90MAC is available in the Canadian Pacific livery. The scale models of these powerful engines are sure to impress everyone who sees them in operation. Pull-out fact (80): GM had to adapt their dynamic brake resistors and fan especially for the AC engine. Because it gets a far heavier train moving, the SD-80MAC had to have much higher capacity dynamic braking. Pull-out fact (90): GM claims that one SD-90MAC can replace two SD40-2's in both heavy haul and high speed freight operations.