Size question | Model Engineer : vimarsana.com

Well at first i thought people are going to go well its 3.5 doh, but since when in life has any size that has been quoted is correct to that size? i have 2 size 9 shoes that are different :-D
so i am taking 3.5 to be inside the rails and nothing to do with wheel track and also my question i suppose is 3.5 is it 88.9mm or is there an accepted figure allowance of that?
@Jon, thats mint, you have no idea how long i spent trying to find that type of info, brilliant thank you.
@noel, i wonder what dictates too tight with 4 8 , is that the radius curves ? surely you could narrow the wheel track,