That’s exactly what they’re doing. A few people have asked questions about future quadruplication, and they’ve stated that adding space for a fourth track would require property acquisitions, so it’s not needed. They’re so afraid of NIMBYism and the Sun Herald they’re throwing out the PTV Network Development Plan.

Knowing the Level Crossing Removal Authority it’ll be a three track wide trench with three platform stations, and no room for expansion.

Hawthorn, Glenferrie and Auburn can all accommodate the 4th track without demolishing the existing stations as Hawthorn had a station yard and space hwas been left for the track when the rest of the yard land was sold off and Glenferrie and Auburn were designed for 4 tracks. The biggest problem is getting a modern straight platform at Glenferrie, where most expresses stop because of Swinburne and the major shopping strip, it might require building the platform over Glenferrie Rd.

Converting three platforms and three tracks into four tracks with just two platforms looks to be the only way you’d squeeze through the likes of Glenferrie and Auburn – good luck with building over the top of the existing shops though!

The extra platform at Camberwell can go where the yard is now. There is plenty of room between Camberwell and Burnley for four tracks, provided that the upgrade is designed so that the outer pair of tracks are designated as Express tracks (no platforms needed) and the inner pair become Local tracks. From Camberwell to Box Hill it’s a bit more awkward, and the additional track would have to sometimes go to the north and sometimes to the south, but it’ll fit. The best approach would be to combine that with grade separation of the two remaining level crossings and bury the line between Mont Albert and Chatham, combining Mont Albert with Surrey Hills as the two stations are quite close together.

Melbourne’s rail infrastructure is held back by four things:
– a lack of an innovative and competent public sector that is willing to look at best practices from overseas (particulary non-English speaking countries like Switzerland)
– a refusal to use more flexible track geometry (e.g. curved turnouts) and bidirectional signalling
– a lack of political will to just get the job done already
– a mass media organisation that criticises any kind of expenditure on the public good, churning out fear-mongering headlines and buzzwords that sway public opinion into NIMBY-ism

As for the capacity constraints – it doesn’t really matter if you are ‘throwing away’ track capacity by running express and stopping trains, if there aren’t more passengers waiting to use the trains. That applied back in the 1990s, but not so much today.