Why? The vast majority of people using a Tulla train would be heading to the city or suburban Melbourne, as most people going to the country would take a direct airport bus service that goes directly to regional centres and bypass the inner city. For the few people wanting to travel from the airport all the way into the city before heading out to the country, there would be plenty of opportunities to change trains.
I would say a Tullamaine airport rail service MUST call at Southern Cross Station !
Look at The Tube. The Piccadilly line from Heathrow bypasses every country rail terminus on that side of town, then heads through multiple inner city stations before finally passing through Kings Cross, the rail terminus furthest from Heathrow.
It's true that the Heathrow Express train goes direct from the Airport to Paddington, but only business travellers or rich people use that. The vast majority of ordinary riff-raff just use the much cheaper Piccadilly Tube line and they have no trouble changing trains at junction stations.
There is no magic one way to build an airport line. Every city is different and hence each solution is different. Just look in our own country for two very different examples of how to make it work with Brisbane basically extending the GC line to the Airport and hence benefits from GC Interurban rollingstock and often light to modertaely loaded trains to the airport as it doesn't go anywhere else. Where as Sydney jumps on the frequent bandwagon because the line is part of the inner core and a 3rd example being built in Perth and Adelaide if it was to get one would be LR and hence different again.
Airport workers are not a keystone of Airport Railways as they are by far a minority, but can equally use the line as per the air traveller on the express and the above comment is a myth.
For Mel the decision needs to be
- Is it a dedicated Airport service and hence have specially fitted rollingstock suited for people not familiar to the city, luggage space etc. If so then the train would start-finish from Southern Cross.
- If its a through train basically feeding another Suburban line using standard rolling stock then Southern Cross is no longer essential but desirable.
If Melbourne had electrified regional services, I would have tapped into the Geelong line aka Brisbane as it would provide regional rolling stock which should come with toilets, better seating and potentially some luggage storage space and frequency.
For Melbourne's case I strongly lean to a dedicated Airport service and specalised rolling stock as the route is fairly long and if running express will not service much commuter traffic although this doesn't stop a few new stations being built and serviced by the same train.