Rebuild the line from the ground up within the existing corridor removing undulations in the track and easing curves using heavy rail, applying suitable super elevation as needed, deep ballast and heavy concrete sleepers. The limitation will then be purely driven by the curve radius.
Logical suggestion, but it would be interesting to learn how much of this has actually already been done. Sections of the line are signed at 140 (XPT presumably) which means they must already be up to a pretty high standard. Such a section for example is Dapto-Albion Park which is tabled for 7 minutes and it's similar in distance and track profile to Canning Bridge-Bulls Creek in Perth which is tabled at 4 minutes (max speed 130).
Anybody who rides the south coast passenger services knows why - the trains move along like a slug on dope, even when there is no infrastructure constraint, no other trains in the way and the section is signed for high speed.
This is presumably the slowdown for on-time running, still taken to almost petulant extremes a decade later. How many more centuries is this state of affairs supposed to last? Is anybody in the railway hierarchy going to go to the Minister and say, hey it's taken a decade but we finally have our act together now and think we can ratchet up the train speeds to shorten the journey times? Or, we can do this if you allow us to undertake this and this work. Or is everybody asleep and doesn't care, or spends many man-hours assembling reasons why nothing can be done as meanwhile the RMS relentlessly surges ahead with its motorways and the residents of south coast rage but don't get any response because they're not swinging seats.
It's like the NSW railways have died, no sign of life, pride, sense of achievement, wanting to move forward, innovate, or anything.