Clearly GP means General Purpose. But what does SD stand for?
Am I correct in that all GPs are B-B, while all SDs are C-C?
SD is Special Duty and yes they are CC. When pulling a heavy load at low speed, the DC motors will overheat after a certain amount of time, so if a four-motor unit can’t get a train up to a given speed in a certain amount of time (I think about 25 MPH), the power must be cut back. By spreading the work to six motors there is less load on each motor, so a six-motor locomotive would be the choice for heavy hauling (Special Duty). The AC motors in newer units don’t suffer from this problem. Others on this forum can give more details on this or correct me.
It should probably be mentioned that SD hasn’t really “stood” for Special Duty since engine horsepower became so great as to demand six motors for practical anti-slip capacity and cooling-system accommodation, and truck design became sufficiently advanced to permit reasonable mainline speed without trouble. It then became the code for a six-motor unit, bearing no more meaning at that point than “E” meaning an Eighteen-hundred horsepower passenger engine or “F” as Fifteen hundred.
This was fully established by the time of the GP40 and SD40, which you will note are both famous locomotives but very different in their use. While there was of course some subsequent development of high-horsepower B-B locomotives, changes in C-truck design (particularly radial steering, which is a difficult thing to implement and maintain on a freight B truck) and operating flexibility for an increasingly high locomotive cost (much of which was independent of the number of axles) combined with changes in the railroad industry itself to make C-C locomotives the only choice for new power.
Note that the SD is retained for EMD power that is given A-1-A or A-A-1 B-1 (see following post) trucks. Here the definition becomes basically a C-frame pair of trucks without regard to the actual population of traction motors.
I have observed that the six-axle EMD’s with four AC traction motors (all on BNSF so far) have a P4 suffix in the model designation. Or is this a designation applied by the enthusiast community?
That might just be a BNSF designation, because I only see P4 and P6 appended for them. Or, the P6 is silent. 
Also, it’s A1A or B1-1B. No such thing as an AA1-1AA to my (admittedly limited) knowledge.
You’re correct, of course; I got carried away with the separate axles and didn’t read carefully before submitting. Note the point about the idler axles facing in rather than ‘out’ as was the common practice for idlers in early electric practice (probably on the theory they acted as leading axles in ordinary locomotives). There are reasons why this is so.
This will hinge on what the “P” is intended to stand for. A “P6” is simply a standard six-motor unit, unless someone retrofitted motors to the idler axles in the ‘special’ trucks and disabled the weight-transfer mechanism and there was a need to distinguish the two ‘subclasses’ of six-motor power for parts or support reasons.
Reminds me a bit of what some of the cabless DD units UP built were called; once you had units with cabs they were DDA but that didn’t make the cabless engines “DDBs” – they were the straight original.
A SD70ACe-P6 has one inverter per axle. The standard SD70ACe has one per truck.
I believe the SD70ACe-T4 comes standard with one inverter per axle, so the P6 designation is not necessary for it.
“Note the point about the idler axles facing in rather than ‘out’ …”
I am not an engineer, so what does “facing in” mean?
Toward the center of the locomotive, away from the ends.
At one point I thought this was related to traction-motor position in the truck frames and clearance to the front and rear bulkheads of the tank (and any other underfloor equipment in that area).
General wisdom is to have all the nose-suspended motors in one truck face the same direction, as the thrusts on the suspension are all ‘in sync’ and there isn’t a weight-transfer effect as there is on ‘trimount’ trucks (when you see unequal axle spacing, it means the motors were kept inside the truck wheelbase; they’re ‘facing each other’ in the longer space between axles, and the opposing pair turns and exerts torque in opposite directions, which causes suspension issues).
Regardless of the way a particular builder or railroad has the TMs facing, if you adapted the truck to have an idler axle I thought it was reasonable to want that to be the axle nearest the tank rather than the one nearest the steps.
Note, however, that there is a long history in electric-locomotive and later, early diesel-electric design, of implementing ‘idler axles’ in trucks as leading (‘outboard’ on a bidirectional unit) axles, possibly by analogy with steam-locomotive design principles.
SD doesn’t stand for special duty anymore. It stands for “Standard Duty”.
Maybe it’s same-old duty. 
I thought I remember reading that the idler axle was chosen as the one nearest the tank because that was the axle where the motor was suspended outside the wheelbase.
One reason for puting the idler axle at one end of the truck rather than the middle is to create an asymmetrical weight distribution. With a symmetrical weight distribution, the front and rear axles have the same inertia with respect to truck hunting (i.e. same resonant frequency) and thus the forces add. With an asymmetrical truck weight distribution, the resonant frequencies for front and rear axles will be different and thus reduce hunting.
Look up the PRR’s development of the DD1…
You are correct and I should have recalled, however the SD70ACeP4-T4 reffers to the B1-1B version (which is also a 4 inverter scheme)
What I’m not sure about is if the original SD70ACe-P4 used 4 inverters or 2.
That would be SOD, wouldn’t it?
Well, I thought I had successfully cheated by hyphenating “same-old.”
Tough crowd …
[:)]
Stick around, L. O. We’ll make a nit picker out of you, too.
Railfans? Nit-picky? No way.
If you can’t pick a nit, can you count a rivet?
You think a mere hyphen would stop me from the SOD line?? (kids today!)