We are talking $$$$ squillions to build SG railways in the SW of WA to what benefit. The existing NG can move everything that needs to be moved. OK build a few new NG wagons/locos if you like but to build new SG would require a huge Pilbara like traffic task.
Just look at what moves on modern NG in Bananaland.
Assuming that the closed and inactive connecting lines won't be reactivated, the South West is an isolated section of 1067 gauge with an end on connection to dual gauge at Kwinana.
I thought that as part of regular maintenance, dual gauge sleepers could be laid on the line to Bunbury and later to Collie and the surrounding mines. Such sleepers are in use on the lines East of Geraldton, apparently not causing any difficulties.
But the locomotives in particular are too heavy to be used on much of the other 1067 lines in WA, with the exception of the iron ore lines out of Geraldton.
The aluminium and related industries have been consistent business since the early 1960s. As far as I know, only a few of the earliest bauxite wagons were taken out of service and these were sold to New Zealand for coal traffic in the South Island. If the track gauge alone was altered, with minimum other changes, locomotives and wagons could be moved to and from the area, and the line could absorb some of the locomotives and possibly wagons being released from the Esperance and Kwinana ore trains.
As it is, last time I was in Merriden, just over six months ago there was a complete Aurizon operated, CFCLA owned Kwinana ore train, more than two years after it was last used. Soon, Avon Yard will be full of ore wagons just as it was full of grain wagons after CBH took over the grain traffic.
Alternatively, perhaps UGL could be prevailed upon to provide new narrow gauge bogies for some of the ACs, ACBs and 6020s, of the type fitted to the PHs. The PHs are the same weight, apparently, so it might not be a big problem, and the clearances should be acceptable on the South West. This would release ACNs for further use in Queensland if required.
If energy costs and emissions concerns closed the Aluminium smelter on Cockburn Sound, virtually everything in the South West would shut down, although some Bauxite and alumina might be exported.
Western Australia has been involved in mining for many years and you don't have to look far to see closed mines, abandoned (or nearly so) towns from Wittenoom and Marble Bar in the North to Ravensthorpe in the South. Some towns, like Goldsworthy, have completely disappeared.
With a common gauge, redeployment of rail assets would be much easier. This applies to new industries in an old area as much as the closing of established industries.