JB, pic #4 in my 'what's in the biscuit tin?' collection (post #106 above, click on it a couple of times to enlarge) shows an NCO triangular track gauge with P48 and On3 locating pins for code 125 (another one for 148) rail. I haven't examined whether
true O-scale code 100 rail can be used.
Top row in the middle of pic #4 is a custom P48/On3 gauge, rail spacing on one side, wheel spacing on the other.
Mike Calvert brought these back from the US and I had thought about holding these as a sort of 'tool hire' set, but I think I need to hand over these ideas to enthusiastic youngsters ...
Jason