This could be a boon for Ballarat, if it's done right, they caution.

Much of the debate has centred around the Melbourne Airport Rail Link, promised by various governments - basically, it's a train to the airport from Southern Cross, potentially with a tunnel underneath the inner-city.

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This tunnel would provide two extra tracks in and out of the city, and between airport express trains, regional trains could also use the tracks, relieving congestion on the metropolitan network and other infrastructure.

On Monday night, the Committee for Ballarat, a lobby group representing the city, held a forum about the future of regional rail.

The Rail Futures Institute, an independent think tank, presented its argument for why the rail tunnel needs to be built.

READ MORE: Government claims fast rail still the aim but opposition grows to non-tunnel option

It was supported by Luke Fraser, project lead for the Stronger Together design group - Stronger Together was a report from the City of Greater Geelong, with the City of Ballarat and other councils from across the state, calling for fast rail in the regions to assist decentralisation and ease the city's population growth.

In attendance were luminaries from the City of Ballarat, including councillors and council staff, representatives from a variety of industries, Wendouree MP Juliana Addison, and other rail advocates - the takeaway was that a united voice now will help get the tunnel under way.

According to Rail Futures Institute president John Hearsch, the tunnel was the physical part of the puzzle - connecting the airport to Southern Cross station via Sunshine involves travelling through an already congested part of the network.

Using existing tracks would mean airport trains would be slow, and growth in outer suburban areas like Melton, and Wyndham Vale and Tarneit on the Geelong line, would not be supported by the number of train services needed to meet burgeoning demand.

Instead, tunnelling from Southern Cross to western Footscray, then connecting to Sunshine, would give airport trains and regional trains a clear run.

The Regional Rail Link, finished in 2015, was already almost at capacity, he cautioned.

That's backed up by Mr Fraser - fast rail to the regions, using airport rail infrastructure, would help "escape the growing suburban crush".

"The other options being floated around are not options at all - for example, Rail Futures Institute has experienced technical operators with decades of experience on the Melbourne network. They confirm there is no capacity to use the current tracks instead of a tunnel," he said.

"The real killer for any other solution than a tunnel is future proofing. The Melbourne Airport Rail Link tunnel will take up to 22 trains an hour.

That will service Ballarat and many other places for many years, but what happens if more daily fast rail services are needed for growing demand in the longer term?

"It's easy to drill another tunnel. But it's impossible to build a second skyrail or reclaim half of a suburb to use the existing track alignment.

"So, in our view and many others, a tunnel is really the only responsible and efficient solution."

Mr Hearsch said that final point is important in a practical sense.

"With the tunnel, there's no property acquisition, you can optimise the alignment, go in a straight line as you possibly can, and importantly, there's no stations, it's a tunnel to West Footscray then you go up to surface," he said.

"That takes so much of the risk out of the construction."

Other work is still needed - Public Transport Users Association Ballarat convenor Ben Lever, who was also at the forum, said the lines between Melton and Sunshine still need to be doubled and electrified.

"The highest priority for us is the electrification for Sunshine to Melton, but adding more capacity to that inner section is important too," he said.

"The fantastic thing is that we're in a position where everyone knows this is so important, and knows it needs to happen, but it's getting it to happen that's the challenge - the last few terms of government, on both sides, have seen a lot of progress, but there's just so much catching up to do."

But tunnels are expensive propositions, as recent history shows.

The airport tunnel is part of a plan from a consortium that's prepared to stump up $7 billion to build and run it, while the federal government has put $5 billion on the table - it's now up to the state government.

The state government has consistently said it is reviewing all options.

"We're working closely with the Commonwealth on all options for airport rail - that will get people to and from the airport quickly with minimal interchanges," a government spokeswoman told The Courier at the weekend.

"All options being assessed for the airport rail will stop at Sunshine for connections to Victoria's major regional rail lines, such as Ballarat."

Mr Fraser said the tunnel plan, should it go ahead, would underpin fast and efficient regional trains, which "in turn redistributes some population away from Melbourne but keeps these people firmly connected to the Melbourne labour market for two way opportunity".

"We realise there are some big competing project priorities that have been sketched out by the state, such as the Suburban Loop, but we are confident the economic and commercial viability of this project is far superior - it will transform Melbourne and the regions."

He added there was no "plan B".

"Melbourne's population grew at nearly 3 per cent last year - almost double the rate of any other 'world's most liveable' city," he said.

"We have to find a way to house these people that is better than dumping them in far outer suburbs with little infrastructure and compromised access to opportunity.

"Once you are clear on this problem, you soon see that MARL tunnel and genuine high frequency fast commuter rail open up a completely different settlement plan for the state - better economic growth and opportunity to regions and far less congestion and sprawl for Melbourne.

"If you do not find a way to segregate country trains from suburban trains, as Melbourne's booming outer suburbs grow, all the extra metro trains will strangle regional services off the network.

"Ballarat and other places will be less connected to Melbourne, not more connected. This isn't a political matter, it's just a fact about what we know will happen if we don't untangle the system."

The Stronger Together report was labelled "not achievable" by the state government soon after its release last year, which left Mr Hearsch "mystified".

"(Success in other major projects) only comes from encouraging local groups, like local governments, to come together as a coalition to present a unified voice," he said - he pointed to the Gippsland line as an example of success, and the $500 million Ballarat Line Upgrade.

The Stronger Together report isn't done yet though - Mr Fraser said a business case has been developed and is being peer-reviewed by "world-leading fast rail experts in the UK".

The Committee for Ballarat's chief executive, Michael Poulton, has consistently said the tunnel is critical for Ballarat's future.

He said representatives from the committee and City of Ballarat met with the state Transport Minister, Jacinta Allan, this week, and three key principles were agreed on - Sunshine is the hub for regional rail, a rail link to the airport will be built, and development of further rail capacity to the west of Melbourne must assist the growth of regional Victoria.

He said there's also been a meeting with federal Minister for Population, Cities, and Urban Infrastructure, Alan Tudge.

"He supports regionalisation as a way of managing population growth in Melbourne and sees regional rail as a way of enabling this," Mr Poulton said.

"In the discussions he has had with the state, there has been no indication the tunnel is off the agenda."

Without the tunnel, Mr Poulton said, "regional rail will not deliver what it needs to cater for the growth".

"Lack of capacity will result in a congested rail system, that will look very similar to our road network," he said.

"The tunnel enables the greater capacity at the Southern Cross end of the line, required to ensure our rail system isn't choked by congestion."

Ms Addison declined to comment for this article.

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