EMD GP-N and SD-N models have the same engines (and many other common components), and are catalogued as alternative applications of the asme powerplant. No railroad has wanted a 4-axle high horwepower unit from EMD since the GP60, and none wanted low-horsepower 6-axle units in the 1970s(*), so there was no "SD15-1," but in general the two types have been available as model pairs. Exception: EMD built the 2400 hp SD24 from 1959 to 1963, but there was no "GP24". Perhaps because they didn't trust their traction motors at such high ratings, EMD's 4-axle offerings in this period were the 2000 hp GP20 (there was no "SD20"(**)) and 2250hp GP30 (there was no "SD30" (***)).
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* Well, not factory-built: one CSX-predecessor rebuilt SD35 with non-turbocharged 1500 hp engines for hump pushing, and others have been rebuilt as 2000 hp units.
** No "SD20" built by EMD: some railroads have called their de-turbocharged rebuilds "SD20".
*** Though "Railroad Model Craftsman," sometime in the 1970s, published "imagineered" drawings!