Dad-1 wrote:...You can't rely on any wagon tracking with it's coupling on the centre-line ! ...
Quite. Accurate coupler positioning is a matter of minimising an easily controllable systematic deviation which disturbs the self centering action of pinpoint axle ends and tyre coning. If these self centering actions are allowed to get on with the job with as few perturbations as possible, running is typically fine. (A major surprise in my youthful OO experience was how poor the original wheelsets and bearings of Slater's quite exquisitely moulded wagon kits were. Insufficient self centering action from a miniscule pinpoint and near cylindrical tyre profile.)