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GP is an EMD (Electro-Motive Diesel) term. Locomotives with the designation GP means General Purpose. SD is another EMD term and stands for Special Duty. The numbers are simply a progressive numbering system. GPs had 4 wheel trucks, SD had 6 wheel trucks. EMD switchers were SW or NW. I think SW stood for switcher. I used to know what NW stood for, but don't anymore. General Electric (GE), Alco, Baldwin and Fairbanks Morris had different terms and nomenclature used to describe their locomotives. Not much and others know far more but hopefully this will get you started!
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