Just two years ago, a Melbourne airport train line was deemed a low priority by Victorian Premier Daniel Andrews though at the end of last year, he said the rail line was on the Victorian Chamber of Commerce and Industry's wish list but it should be more than just a link between the airport and the city. This week, the railway project looks set to become a reality.

And it's thanks to Prime Minister Malcolm Turnbull who announced that the federal government would commit $5 billion to building the train line, with the state government having to contribute another $5 billion. The total price tag could go up to as much as $15 billion.

In announcing the funding, Turnbull pointed to other cities, telling The Guardian: "Most great cities with big airports have railway linkages. Melbourne has been left behind by failures to make decisions about the investment in this railway."

Turnbull said he hasn't spoken to Andrews but has sent a "very detailed" letter suggesting the state use the $2 billion it would get from selling its share of the Snowy Hydro Scheme. Private investment would also be needed. The aim, the Prime Minister said, is to commence construction "within a few years".

RMIT University infrastructure and urban planning expert Professor Jago Dodson echoed Andrews' sentiments, saying the rail link news is welcomed but it needs to be part of a broader transport plan if it is to have major benefit.  

"The proposal by the Federal government is welcome news from the perspective of improving Melbourne's rail network and its integration with a major activity node," Dodson said. 

"However, a big concern is that the Airport link is happening outside of any clear transport plan for metropolitan Melbourne, because the Victorian government has no transport plan.

"We have no clear formal assessment whether the $15 billion to be spent on Airport rail is preferable in terms of costs and benefits to commencing the second metro link, or expanding suburban rail access to underserved growth areas, or other public transport improvement.  

"And the eventual route needs to include opportunities for urban redevelopment beyond rapid airport access. For example, an Upfield alignment could enable redevelopment of the former Ford factory site." 

Dodson warned it would be foolish to steam ahead with a new rail link without considering the bigger picture for Melbourne's already heaving public transport network. This includes limiting road use.

"We need to expand the Melbourne metropolitan rail network far beyond the Airport line if we are to meet the needs of a city of eight million, but we also need to limit road competition with this network," he said

"It is foolish to expend $15 billion in rail development if we then also build major roads, such as the Westgate tunnel or Tullamarine widening, that compete with our rail investment".