Ever heard of MREX?
I have. 
With respect, it’s the wrong proposal for Flagstone/Beaudesert. That corridor is classic longer-distance suburban rail, it needs decent baseline frequency off peak plus somewhat higher frequency in peak, but it doesn’t need its own CBD corridor. It’s also already a heavy rail corridor.
Plus, we have sufficient CBD capacity on the QR network from the Southside for a long, long time to come.
First, Gold Coast + Beenleigh combined will only use about 2/3rds of the trains/hour. Then there’s the option to run 9-car trains through CRR to trade off between running more trains/hour and more train per train. Then there’s the option to quadruplicate from Boggo Rd south and put either the Beenleigh or Beaudesert lines over the Merivale Bridge (if we break sectorisation we can even do it with the current 3 tracks, I guess).
So where is the right corridor for automated metro? Well…