Are you contemplating hex editing the 3DP files to lengthen the F-unit body by about 4 feet, to create a true FP? It wouldn't be a difficult project; just add about +5.0 to all the Y-values of the points that form the rear end. The slightly harder part would be doing the same thing to the rear truck and all of its bogies.
It isn't on my radar screen right now, but I've long ago thought about creating E-units via this method (they'd require three axle trucks, so it would be a little more involved).
Of course, you can do the skin without doing the body, and make the body edits later (or bribe someone else to make them for you).
Did any of the Cannuck railways run with A-A sets rather than A-B? I can tell you how to create a backwards-facing engine, too; it's pretty easy, actually. A back-to-back AA set would be rather unique.
=Winchester, Paston & Portsmouth=
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