http://fmgl.com.au/UserDir/Documents/Fact%20Sheets/FACT%20SHEET%20Rail%20Expansion%20-%20Sept%202011.pdf
8.4 billion builds about 250km according to this FMG fact sheet. With 120km of that just duplicating next to existing rail.
Add a bit of CPI and labour shortage by the time AGO get around to it same may cost $10bn. Only my thoughts.
I'd still will invest more into AGO but I just have a feeling they will need to raise cash for this rail.