steve-in-kville
Okay... from the pictures online it looks like there are four axles per truck. Makes sense.
As part of your education, I recommend that you read Lionel Wiener's Articulated Locomotives with a particular eye towards his 'taxonomy' of various approaches and his proposed notation. That is where the le Massena kerfuffle about using the plus sign for an articulated joint (as in 2-6+6-4 instead of 2-6-6-4 for Whyte-coding a simple articulated with Mallet-style articulation, or in a different system that counts axles, 2-C+C-2 instead of 2-C-C-2 for a GG1) that caused so much confusion and delay in the 1970s came from.
Now on reflection I may have been using the convention almost precisely backward, but we will take up why this might be a failure in a bit: let it be illustrative of the difference here.
The Flexicoil three-axle truck you understand, and a locomotive with two of these that has a fuel tank between them is properly C-C when all six axles are motored.
EMD in building two-motored DD35s and 40s could manage four motored axles in a rigid sideframe (essentially like an extension of a three-axle Flexicoil frame, although different in detail design) and hence we can correctly call this a D truck. (Some European systems distinguish conjugated or coupled axles, so a "D" truck would have to have the wheelsets geared or rodded together, and one with individual noise-suspended traction motors would be denoted "Do", but we don't use that convention here yet...)
Now Mr. Goding built his more flexible truck by taking a couple of shorter-wheelbase Flexicoil C trucks and lopping one outer axle off each (which converted them into B trucks) and arranging a revised pivot location. He then had to mount these to a typical 'older' SD chassis that still used pin swivel truck mounting, so he used a longitudinal beam, called a span bolster, to link the two trucks together but still allow each to pivot around its own center pin location. The span bolster has its own pin connection which links to the locomotive frame just as a regular truck would do.
This is the critical difference between a "B-B" truck and a D truck: the ability of the 'halves' to pivot independently.
Now for the fun: there have been locomotives with multiple B trucks in which not all of them pivot: a couple of the PRR experimental electrics of the early '50s were built that way and, more interestingly, a locomotive Mr. Goding knows, the 'white wonder' GM10B. Here the center truck has considerable lateral play, but is not articulated to its neighbors, so we can call the locomotive a B-B-B all day long and be justified.
But when we put the two trucks on the span bolster we effectively articulated them, just not directly 'frame to frame' as in true truck articulation (which, remember, is what that plus sign is supposed to denote in Wiener). And just as on the original C-C version of the SD, there is a fuel tank between the two trucks still, so the result is not a B-B-B-B and can't really be a B-B+B-B as the two span-bolstered assemblies are not linked; it is a B+B-B+B ... reaching for the aspirin bottle yet? ... if we want a consistent notation that tells us how the driving axles are mechanically arranged under a locomotive.
At some point we should probably develop a notation for rigid-frame radial-guiding trucks like the HTCR family; we might even have a convention for those with steering levers vs. those without. Wiener took up the issue (there was a whole set of steam locomotives that had 'radiating' axles, some of which accomplished the feat with side rod drive!) but the result he arrived at in 1930 has not been generally adopted (or recognized, at least) and may be too weird for normal use.