You’ve seen them. SD-40, E-9, SW-7, GP-60, etc. They are notations that Electromotive Diesel (EMD) has given to its diesel locomotive line over the years. Some of those numbers I have known for years, but some of them I learned only recently from experienced modelers in the model railroad club to which I belong. Let me give you the list I have:
NW (as in NW2, NW3, NW5) stands for “nine hundred horsepower welded frame”
NC stands for “nine hundred horsepower cast frame.”
SW stands for “six hundred horsepower welded frame.” (Surprise! I thought it stood for “switcher”. Apparently SW has come to mean “switcher” as EMD switcher locos of much greater than 600 hp have been designated “SW”)
TR stands for “Transfer”
E (as in EA, E2, E3,..) stands for “eighteen hundred horsepower.” (Another surprise! I thought “E” stood for “Express.” Again, later passenger diesels of increased horsepower continued to be designated “E” apparently for “Express”.) All “E” units had two diesel power plants.
F stands for “fifteen hundred horsepower.” (Surprise again! “F” did not originally stand for “Freight”, but again as horsepower increased to 1750 hp, the designation “F” stuck and could be thought of as “Freight.”)
With the original FT’s, the “F” did stand for freight. The "T" stood for “Two-thousand seven-hundred horsepower” as these units came two 1350 horsepower units permanently coupled together, an A-unit with a cab and a B-unit which was cabless.
GP stands for “General Purpose”, and is the designation applied to any EMD loco with exterior walkway, riding on 4 wheel trucks, meant for use in mainline or branch line service but could also be used in the yard. The exterior walkway made switching with this locomotive easier.
SD stands for “Special Duty.” This is another designation that has evolved. The original SD-7’s and SD-9’s were meant for branch line service on track that was light enough to require the six wheel trucks to bear the locomotive weight. That has evolved to mean any six wheel truck EMD loco with exterior walkway. The six wheel trucks became necessary now to provide the extra traction needed for the high horsepower of modern power plants. SD’s that were meant for light track service now came to have the added designation of “L” as in the SDL-39’s of which the Milwaukee Road was the sole purchaser.“DD” as in Union Pacific’s DD-35’s and DD-40A’s had to do with the size of the truck. “B” is a term used for four-wheel trucks and “C” is a term used for six-wheel trucks. It was only natural then that “D” would be used to designate eight-wheel trucks, and “DD” units rode on two eight-wheel trucks. All “DD” units had two diesel power plants. The DD-40A is said to be the most powerful diesel locomotive ever built. Union Pacific and Southern Pacific were the only railroads to own DD units.