Re the proposed back to back gauge being proposed. Note that the min-max of 12.40 to 12.50mm applies ONLY if you have the matching wheel profile - which is that proposed by the BRMSB as ‘EM’ in 1950. This profile was used by Alan Gibson and MayGib and sold by them as ‘fine scale OO’ during the time I was developing and producing my range of O14 kits. I took wheels from both, the loco tyres coming from Alan and those used in the rolling stock from Ken Pelham at ?MayGib. I moulded my own centres in polypropylene - the rolling stock axles were purchased jointly with Alan Gibson in batches of 20,000 (split two ways). Alan sold them for ‘ TT fine scale’.

Use of any other wheel profile will cause varying problems - good tracking is only assured if standards are kept to quite strictly - particularly on locos with more than two axles. Mixing profiles, especially those un-published is risky.

My standards were the result of practical experiment - based on the old BRMSB measurements and logic. They allowed perfect tracking of small four wheel locos and wagons around sharp radius curves without derailment, either pulled or propelled. A long time ago I did propose that, for models of larger two-foot gauge prototypes, it would be better to adopt modified NMRA ‘Sn3’ standards - which are more sophisticated than those of the BRMSB - which were more guidelines.