That's what I was thinking, that the centers remained the same if the stock remained the same. I confused the issue by mentioning loading gauge since obviously more serious modelers are modeling wider stock, although I think I will be running rtr OO stock, Murphy, Provincial etc. Just to clarify things though, models like the MM cravens for example are modeled correctly to scale widths except the bogies/wheels, correct? Ivan, thanks for those links,some more reading to be done.
I presume point work on the 21mm has to be much longer than in 16mm for the same reason you cannot have curves that are as tight?