National guage standardisation - why 4'8.5"?

["Bradley Torr" <truenorth@one.net.au.SPAMTRAP>]

I just have a question that's bugging me - when PM Andrew Fisher started
building the Trans-Australian Railway in 1912 (or whatever year it was),
why did the Commonwealth Railways engineers choose 4'8.5" for the railway,
and all subsequent CR projects, right up to the Melbourne to Adelaide
standardisation in the 1990's?

Out of the five mainland states, two had 3'6" (WA and QLD), two had 5'3"
(VIC and SA) and only one had 4'8.5" (NSW).

I would have chosen 5'3" had I been a CR engineer way back then. Why?
Because two states already had it, and 5'3" from Port Augusta to Kalgoorlie
would have been a logical extension of South Australia's system. It would
have been possible to travel all the way from Orbost VIC to Kalgoorlie WA
on the one guage as well. Also, it seems kinda unfair for a guage used only
by New South Wales to be imposed on the rest of the nation.

Regards
BT