I believe that was dependant on a turnback being built at Gowrie as part of the “Wider Network Enhancements” package, but it got descoped.

https://wongm.com/2022/11/wider-network-enhancements-and-the-metro-tunnel-project/

The section of third track at West Footscray isn’t particularly long, with the section behind the centre platform being designed as a siding for a ‘hot standby’ train to wait pending deployment towards the city.

Using the Bunbury Street tunnel was proposed as an alternative to building Regional Rail Link in the 2010s by the likes of Paul Mees, who believed we could just sweat our existing assets harder rather than building more tracks.

https://australasiantransportresearchforum.org.au/wp-content/uploads/2022/03/2010_Mees.pdf

It has track connections to both tracks, at both ends, which would allow express services to overtake stopping all stations services in peak direction. Obviously that would delay the stops all train unless the third track was extended towards Tottenham, but in ten years it would be a great political pledge:

“Faster Airport trains for free”

Surely a vote winner! Just don’t tell the thousands whose journey became 2 minutes longer…

Technically, you are correct, but if there are issues beyond West Footscray, then the turnback would be required, so even when the line to the Airport is finished, the turnback would still be needed in case of emergencies.

As for what happens with electrified Melton and West Werribee line trains – the Melbourne Airport Rail business case considered thee options:

https://bigbuild.vic.gov.au/__data/assets/pdf_file/0007/710179/MAR-Business-Case-Main-Document.pdf

– Option 1: Metro Tunnel – connecting to the CBD via Sunbury tracks and the Metro Tunnel.
-­ Option 2: Regional Rail Link (RRL) – connecting to the CBD via the existing RRL track pair to Southern Cross Station.
– Option 3: Sunshine Tunnel – connecting to the CBD via a new tunnel to Southern Cross Station.

So an extra track pair inbound from Sunshine via tunnel is another possibility.

At Sunshine they’d have to terminate in platform 2, change ends, then head back towards outbound trains a few hundred metres before they get to the crossover back to the citybound track.

So possible, but not really the most efficient operation – so I suspect they’ll just terminate at West Footscray instead.

The eastern suburbs did get their branch lines built 100+ years ago when the suburbs out there were being built, so can’t really say it’s a horrible idea.