It's staring right at you in the face - the north-western section of the SRL will be the airport rail link which will be complete before 2030. Everyone needs to give up on stupid fantasies of the line that will be part of the northern section (likely the last to be built) will be extended anywhere near the west.
You have to look at how the west will develop - and its fate was sealed long before SRL was even a thing - it was back when the urban growth boundary was put in place well over a decade ago.
The base map
is one that shows you the status of any PSP - Precinct Structure Plan (a set of documents that govern how new suburbs are made in greenfields site) - in the West with me adding the red lines to the map for context.
In a nutshell, we need to focus on the Melton and Wyndham Vale corridors. The Sunbury line will only really take on any major new patronge from Sunbury itself, NOT south of Watergardens because the area will only take on higher-densities, rather lower-densities of new fringe greenfields dev.
The west is going to develop very differently to how the east has over the past century and a half. It is a false dichotomy to wish some form of warped equalisation of the east with the west - it's apples and oranges. Each of the new PSPs, broadly speaking, have mixed-use and mixed-density town centres built around existing or new train stations on either the RRL corridor or the Melton line. The east was developed as towns along country rail lines and is fundamentally complete (except for the outer S.E. - which is a focus on new services to Cranbourne/Pakenham).
The tunnel that has been talked about and was in the leaked version of the NDP will eventually be needed to take either both quadruplicated & electrified Melton + electrified Wyndham Vale (maybe also quadruplicated to maintain regional / metro split) into the city and thanks to the exclusivity of a train pair for those two suburban services, frequencies can scale up and down as needed, thus forming the southwestern corner of the loop but also
acting as a rapid access pathway to the city (this is a clear and present advantage for the west over east going forward).
We already have commitments to have 2 of the 4 sections complete by 2030 (Southland-Box Hill and the Airport line), I'm willing to bet the Melton/Wyndham Vale electrification (either both together or phased) will be the 3rd to go under construction with the 4th phase the second part of the new rail tech extended from Box Hill to the airport.