Duplicating Melbourne’s least-reliable railway line through the city’s growing northern suburbs would cost the taxpayer just over $245 million – a figure the Greens have likened to the removal of a level crossing.

Building a second track on the Upfield line past Gowrie station would cost $246 million, and a less-expensive “turn-back” facility that could also boost services would cost $54.5 million, Parliamentary Budget Office costings, commissioned by the Greens and provided to The Age, show.

The figures do not include ongoing operating and maintenance expenses, which would cost the state budget about $49.9 million over the next decade.

Commuters on the line have had to deal with far less frequent services than those on other parts of the network because the line runs on only one track past Gowrie station.

Trains heading north past Gowrie must wait until the track is clear to continue their journey. Duplicating the line would provide significantly more capacity for services.

Passengers face waits of 15 to 20 minutes during morning and afternoon peak periods. The government’s latest budget commits $98.7 million to boost the Upfield line timetable to a minimum 20-minute service every day, including weekends.

Passengers on other lines such as the Mernda, Sandringham and Glen Waverley lines, wait less than 10 minutes in peak hour, and the Cranbourne, Pakenham and Sunbury lines will have similar frequency once the Metro Tunnel opens.

The Parliamentary Budget Office’s $246 million costing includes track duplication between Gowrie and Upfield stations and construction of an additional platform at Upfield.

A cheaper improvement to the line, also costed by the budget office, shows building a turn-back facility at Gowrie allowing trains to turn around at the station and head back towards the city, would cost $54.5 million. This would also provide significant timetable improvements.

This upgrade was included in the business case for the Metro Tunnel. However, the government is yet to commit to implementing it, as it will decide where to make further rail network improvements once the Metro Tunnel is finished later this year.

Brunswick Greens MP Tim Read said the cost of duplicating the Upfield line was comparable with a level-crossing removal and would make a huge difference, at relatively low cost, to those living along the line.

He said the figures show why upgrades to the line must be prioritised, as it has been selected to host two of the government’s activity centres, where taller apartment buildings will be encouraged.

The government says its proposed activity centres are slated for areas already well-serviced by public transport and are therefore suited to higher-density living as there will be minimal pressure on infrastructure.

Read said: “With new activity centres planned for this line, I do worry that the main activity in these centres will be waiting 20 minutes for a train, unless the government duplicates the last leg and increases frequency.

“We have room for more people, but we don’t have room for more cars, so the government needs to turn this line into one that people want to use.”

Public Transport Users Association spokesman Daniel Bowen said the single-track section of the line not only limited services but made delays worse when they occurred.

“Frequency, particularly in peak hour, is very poor compared to the rest of Melbourne,” he said. “That area is growing. There’s a lot of new housing going in around Coburg and the other inner suburbs along the Upfield line, as well as development further north. So it’s only going to get busier.

“Duplicating the line and adding more services really does make a lot of sense, particularly with the Metro Tunnel opening.

“That was a constraint with City Loop capacity, but as soon as that opens, there will be plenty of capacity for extra services.”

The budget office’s costings predict a Gowrie turn-back could be completed by the 2027-28 financial year if started this financial year. The duplication project is estimated to need a longer construction time and be completed by 2031.

Read said the duplication was not just about more services, but also increasing access to stations further north.

In June, The Age revealed the government had commissioned and received a report into the growing need to improve rail services in Melbourne’s booming north and west.

That document supported preserving the option of duplicating and extending the Upfield line and dovetailing it with an extended Craigieburn line, with a new terminus station in Wallan, about 60 kilometres from Melbourne’s CBD.

The Upfield and Craigieburn lines would be joined by using a dormant freight link between Somerton and Roxburgh Park.

An Allan government spokesperson said the opening of the Metro Tunnel would free up the City Loop for more services across the network.

“Along the Upfield Line we have already removed five dangerous level crossings, with another eight to go and this year we have invested to make sure trains run at least every 20 minutes all day, every day – including late nights and weekends,” the spokesperson said.

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