This may have been discussed before but,
Besides the size, trucks, performance, etc of EMD's GP and SD locomotives, I was wondering what a GP locomotive would be used for in comparison to an SD locomotive. I've heard that GP locomotives are used for road switching sometimes. Are they commonly used in mainline freights? What would make a railroad prefer to use a GP loco over an SD loco, or vice-versa? On top of that, have switchers ever served another purpose besides switching, and what does the F in F45 or F40ph stand for? Answers would be appreciated.