This has probably been discussed many times beyond count here on this forum but I was wondering how the letters and numbers like SD-40’s, AC and GP were put into place. What do they stand for and why those particular letters were decided upon.
Hopefully not to dumb of a question from an inquring mind.
Well, as a start, have you seen the “Locomotive Model Abbreviations” article at:
http://www.trains.com/trn/default.aspx?c=a&id=207
It’s in the “Railroad Reference - ABCs of Railroading” section here:
http://www.trains.com/trn/default.aspx?c=ss&id=13
There are other articles there that also discuss aspects of this kind of thing - such as the one titled “Who Built the Diesels” by Paul D. Schneider.
Hope this is helpful.
SD and GP are easy - Special Duty and General Purpose, which aptly described their uses when they first hit the market. Nowadays the SD could be better likened to a General Purpose locomotive but the designation remains.
AC usually indicates a locomotive using Alternating Current (vice DC or Direct Current) to power the traction motors.
Don’t think I’ve ever read what the H in H16-44 for a Fairbanks-Morse locomotive meant.
On the other hand, the Baldwin “Centipede” was properly designated DR-12-8-1500/2 meaning Diesel Road locomotive, with 12 axles (8 of which were driven), and two engines of 1,500 horsepower each. Pretty cryptic.
I seem to recall that the E in the EMD passenger locomotive designators (E-3, E-9, etc) meant “eighteen” as in 1800 HP. Even though the horsepower rose, the E stuck.
The U in GE’s U-boats stood for “Universal.”
The list goes on.
There are many different designations within locomotive model names but I’ll take the 3 examples you mention.
GP = General Purpose- when EMD introduced the GP-7 in 1949 they marketed as a jack -of -all -trades able to handle yard switching, local freigths, mainline express, and passenger trains. Since then EMD has branded all their 4 axle roadswitchers GP’s.
SD = Special Duty- any 6 axle EMD roadswitcher. At the time of it’s introduction the SD7 was aimed at heavy haul applications, particularly low spead bulk (coal, grain, ore, ect.) “drag” service. O.C in the modern era 6 axle freight locomotives have become the industry standard, particularly on Class 1 RR’s.
AC = In modern usage signifies a locmotive using AC rather than DC traction motors (and the associated electrical system). In the very early seventies EMD offered models such as the GP38AC which had DC traction motors but used an alternator rather than a main generator(the latter was standard on the GP38, the alternator was an option until the GP38-2 which used it as standard).
To really understand the naming conventions you need to read one of the better books on the subject…