As promised, here is a photo of the underside of one of my 6-wheel coaches fitted with a Climinson underframe. This was my own design and was drawn up and etched for me by Bob Jones, before I got into CAD.
The outer W-hanger brackets are free to rock on the ends of two arms whose 'tails' engage in slots on the ends of the centre W-hanger bracket, it free to slide sideways and rise and fall between tabs bent down from the baseplate. The pivots of the arms are on cross beams which allow the arms themselves to rock end to end, but keep the whole thing laterally stable. The underframe is therefore fully compensated. Contrary to what I said before, the pivots are actually arranged to be roughly 1/3rd / 2/3rds between the outer and inner axles to try and make the weight distribution even over all three wheel sets. The design allows for a range of different wheelbases.
Prior to fitting these underfames these coaches (which were originally built around the early 1970's with a sliding centre axle) frequently derailed on the 15" radius superelevated curve on my first layout, despit it being fitted with a check rail. After fitting them they sailed round it nae bother!
Jim