"I'm curious as to how K-Line was able to do this."
Williams Alco PA and E-7 diesels also have a proper center-blind model truck design (rather than the awful end-blind design). The end-blind design allows a manufacturer to add 3-axle trucks to a frame without changing the pivot-point and maybe other factors used by the 2-axle ("B" truck) design. The end-blind 3-axle type is actually a 2-axle truck with an awkward decoration sticking out of it.