The Overhead from Cabulture to Gympie is used by interurban services and this will only get busier with time. So an SG up this way will not be DG of tracks used by sparks.
So 170km of stand alone track.
Now north to Bundy, about 200km is used by four services daily RTT and long term this is likely to remain and may even expanded. If you want to use the existing line you have to convince the QLD govt to replace the RTT with diesel and remove OH.
Now next 200km to Gladstone is used by once daily RTT. You could argue this section of OH would be removed and line conveyed to DG as it's mostly realigned and mostly suited to SG.
Yes single stacked containers fit under OH, but DS doesn't.
Now from Brisbane to north you will need 100% stand alone freight line. If you want DS then you need to start from A.R.
So for all this Billion or so, what is the benefit or problem trying to be solved? Without DS may as well gauge change at AR. Cost of introducing DS along coast would be astronomical.
So place the while concept into the pointless bin and focus on real issues.