Piece of cake....
'The reason that the E's went to A1A -A1A from B-B, was that the locomotive weight was getting too high for four axles. The EMD A1A trucks were laid out symmetrically, which was not possible with a three motor C truck. One other reason for sticking with two motors per truck on the E's, was that each engine powered a single truck. Transitioning between series and parallel motor connections was easier with just two motors as opposed to three motors. This was not the same issue with the single engine PA, but Alco's first foray into passenger power was the dual engine DL-109, which had the same issues with transition as the E units.
It was simpler to use re-geared F units for mountain use than converting E's to C-C's'....
WTF it actually means TPBM may know.