jlr3266 wrote:This is a very informative thread. So, SD came about with the first six axle engines and has nothing to do with actual use, as in "Special Duty" versus "General Purpose"?
Well, to be fair, when the GP7 came out (about the time of the F7, or ...F
reight unit 7 series
), it was meant as a (well, to EMD, the
) General Purpose freight locomotive (which it, and it's many 4 axle brethen, served as until the 1970s). Special Duty locomotives, as their name implies, were for special purposes such as hump engines, heavy drag haulers, or light-load lines (where spreading roughly the same weight over 6 axles as opposed to 4 led to a much lower per-axle loading, better for branch lines having lighter track but large radius curves). Hence 2,724 GP7s vs 188 SD7s.
(OK, yeah, the SD7 was in production for 2 years vs 5 for the GP7, so for a better comparison: 4092 GP9s vs 471 SD9s - BTW, F production was getting low at this point: 87 F9As and 154 F9Bs)
And the granddaddy of them all, the E units? E
ighteen Hundred HP. (and only 4 powered axles, albiet they had 6 axles all total)
(I was pulling all this from Wiki, but actually I learned it years ago from my well worn copy of "Diesel Locomotives: The First 50 Years")