Aussie GE units...

BHP's loading gauge differs quite a lot from PR's. They can use off the shelf GE's, BHP can't. Hence the differences with the AC6000.
GE was not interested in doing any mods to the -9 or the 44AC for BHP at the time of early talks, EMD would also not do any mods to the SD70M as it was at the time.
In the finish EMD would do the mods etc for BHP using EDI (Aust. agents for EMD) to finish the loco in Australia and setup the locotrol ATP etc.
BHP also liked the dual a/c unit in the cab (I was told this by the project manager). One point that caught BHP out was the cab not being isolated, like the current order for 10 more is.

Yes it's true that PR has had a good run with the -9 unit, just like BHP has had an outstanding run with the fleet of -8 units.

PR and FMG have new -9 units on order, 10 for PR and 15 for FMG. PR has also signed a deal for 10 new EVO units ex-works late 2007.

Does BHP have any tunnels or bridges that limits the loading gauge? How much smaller or larger is the BHP loading gauge compare to the US loading gauge?

From what I understand, the BHP ac6000cw are massively heavy, I guess BHP must have one heck of axle load.

The answer to the AC6000 Q, the weight is 436,600 lbs, which converts to about 198 tonnes.
BHP work on a maximum of 40t axle load.

The BHP loading gauge is confined to the original Oroville Dam rotary car dumper.
This dumper is just about to be retired and completely rebuilt.
Only Car Dumper 4 (the brand new one at Finucane Island) will take the SD70ACe/LC units, once the other dumpers are rebuilt, then they to will allow the SD70ACe/LC locos to pass through.
The AC6000CW had the cab sides tapered and the radiator lowered and lengthened to allow them to pass through the car dumpers. If BHP had taken -9 units, then similar modifications would have been required to them.

Pilbara Rail has a similar problem, the -9 locos will not fit through the East Intercourse Island dumper. Originally they could, but the cells got replaced with smaller ones. These are either just getting replaced, or will be replaced very soon to allow locos through the dumper again. Hamersley Iron has always used a shunt loco at Parker Point to spot the loaded trains, this will finish soon with the new 'run through' dumper is completed.

The new Pilbara Rail EVO units are being billed as the largest GE locos ever, (maybe a rumour) and they will have twin radiator fans and be 76' long and also have air to air cooling. I'll know more when building starts or they arrive!!

I should have also added that Pilbara Rail's -9 are 434,000 lbs.

seeing these loco's is making me think of modeling one.

Kaslo Shops do the Pilbara Rail cab, it's part no. HC-4.

Then you just need to add the extra air intakes for the Dash 9 units.

The BHP AC6000 is a fair bit more involved, cab sides are on a taper, radiator is lower and longer etc.

Good luck!