You bring up a good question. The answer is, who knows? EMD is known for really arcane nomenclature. The biggest examples:
-What is the diff between "General Purpose" and "Special Duty"? Especially now that no "general purpose" units are built and everything is "special duty", which means it's the definition of "general purpose".
-What does the "70" mean in SD70? It's not the horsepower. Back in the 60's, GM's marketing machine decided to let the model number grow faster than the horsepower, but nobody was fooled. Railroads don't buy locomotives like suburbanites buy cars.
The list goes on and on, but I quit trying to make much sense of it long ago.
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