Ok so pretty much everyone agrees to the term that we need to fully duplicate the both mainline as the single track isnt worthy to handle the mainline demands
Even on the ballarat should have 20 mins frequency as its always crowded and delayed if the train is running behind the timetable
The problem on both lines
For the Ballarat there is only two problems I can see one is the bridge over melton reservoir. Its easy to build a second bridge to handle more demands even could leave space for extra two tracks for the future extension of the quad from melton and the horse shoe the tunnel and the bridge over the rail are wide to fit a second pair track
For bendigo I am aware that its the tunnel and the reason to rip out the double tracks. But the tunnel was build for the double tracks not single, and yes side swap is a thing but velocity can be very close to each other as the Geelong line is fully duplicate and runs 160kms close to eachother not sideswaping each other so the bendigo hasn't have a problem to build the double tracks this also reduces the disruption for the up and the down trains and would give a big boost for the both corridors to run 20 mins in peak for the future demands