Well I'll throw in my two peneth worth have built quite a few six wheelers for my Caledonian fleet I've a favorit a mix of ideas but as a rule I have one outer axle fixed the other end rocking the center axle is designed to slide from side to side by as much as 1.5mm each way.
It's hard to get it all set up so it sits flat with all wheels touching (built and set up on a glass sheet) and I'm finding it hard to get a consistency in production meaning some run realy well and some are pigs.
With this method all will run on 3rd radius curves and Large Peco points bit it's hit and miss when trying to run on 2nd radius curves and medium points with some going through and some not which I freely admit is more down to my build inconsistency than any thing else.
I've thought of different methods including all the above but am not sure which will give the best overall results.
Looking at some of the passenger stock with the springs, axle boxes and step boards it can be hard to actually see the wheel set in detail and I must admit I'm a little bit tempted to try fixing the center wheel set and cutting away the lower part of the flange effectively making the six wheelers four wheelers. It would make for an easier life and would make the whole lot much more trouble free when running and its one of those things that to the causal observer would not be visual if you didn't actually know about it.
One of my builds in question. All very visable while unpainted but once painted and sat on the track the flange method would become all but invisible.
This one will run quite happily on large radius points and third radius curves but becomes very indifferent on anything less.