The tunnel will in turn link the Sunbury rail line, in Melbourne's north-west, to the Dandenong rail corridor in the outer south-east, allowing an extra 24,000 passengers an hour across the train network. The Baillieu government regards the project as one of its top infrastructure priorities after the east-west link, an 18-kilometre road connecting the Eastern Freeway and the Western Ring Road. But a business case ''concept of operations'' report, obtained under freedom-of-information laws, has revealed the urgency of the proposal, which requires state and federal funding to get off the ground. The Department of Transport's June report warns that without the infrastructure the number of ''load breaches'' - where there is an average load of more than 798 passengers in a train - will grow over the next seven years. Projections suggest that by 2020 trains will be overcrowded for at least three hours during the morning peak, and on the Werribee, Sunbury and Dandenong lines, demand will exceed 1200 passengers, meaning many commuters would be left behind.

Illustration: Matt Golding. The Werribee/Williamstown and Frankston lines will start to face ''significant shortfalls'' from around 2015. The Craigeburn, Upfield and Sunbury lines will face significant shortfalls by 2016, and the Sandringham line from 2017. Shortfalls on the Dandenong corridor are ''immediate'' and ''will rise to a severe shortfall by 2020''. The documents also show: ■The rail project ''will need to be progressively implemented over the next decade, commencing in the next two years''. ■Part of the plan involves upgrading the Dandenong rail line with longer ''nine-car'' trains, timetable changes, signalling upgrades and road-rail separations.

■New branch lines to Rowville and Melbourne Airport (the subject of feasibility studies) are being considered as part of longer term improvements to the network. ■The underground tunnel will not be able to accommodate freight trains, meaning existing freight services that operate during peak time would have to be re-timed. Transport Minister Terry Mulder described the Melbourne Metro project as a ''city shaping'' plan that will ''fundamentally change the way our transport network operates''. ''More frequent trains, more stations and increased capacity will also deliver significant flow-on benefits to the rest of the transport system, it will take pressure off the major arterial roads including West Gate and Monash Freeways,'' he said. Opposition transport spokeswoman Fiona Richardson said the government had been too slow to tackle overcrowding.

''The Liberal government has attempted to dampen the fires of discontent among commuters by initiating numerous studies into the Doncaster, Rowville and Tullamarine rail links as well as the Melbourne Metro Tunnel. The problem is, commuters can't catch a feasibility study to work and the Liberal government has given absolutely no indication that they plan to actually build any of the rail lines under investigation,'' Ms Richardson said. Infrastructure Australia, which makes recommendations to the federal government about funding of major projects, puts the Melbourne metro rail project at the top of its priority list as one of Australia's most urgently needed projects. The project has been deemed ''ready to proceed'' and is in the planning approvals stage. Public Transport Victoria says that if planning approval is granted, the start of construction will be contingent on Commonwealth funding, but it is refusing to disclose how much the project is likely to cost. A similar project for a tunnel from Footscray to Caulfield was proposed by the former Labor government and costed at $4.5 billion for the first stage. Labor's track was about 17 kilometres; the Coalition's tunnel is nine kilometres. About $90 million in state and federal funds has been allocated so far. PTV also admits construction will cause some disruption across the city, but the extent will depend on the precise tunnel route and station design. Community feedback will be sought by the government next year.

THE PROBLEM ■ Train patronage has grown by 70% in the past 10 years, and 40% in the past five. ■ Melbourne’s population is expected to reach 5 million by 2025. ■ By 2020, trains on all lines will carry 900 to 1200 passengers in peak hour, resulting in ‘‘significant overcrowding’’. ■ Crowding will be so severe on the Werribee, Sunbury and Dandenong lines that some passengers will be let behind.

■ The Werribee/Williamstown and Frankston lines will face ‘‘signii cant shortfalls’’ – where demand exceeds capacity – around 2015, followed by the Craigeburn, Upifeld and Sunbury lines in 2016, and the Sandringham line by 2017. ■ Shortfalls on the Dandenong line are ‘‘immediate’’ and will be severe by 2020. THE (POSSIBLE) SOLUTION A nine-kilometre tunnel under Melbourne to link the Sunbury and Dandenong lines.Five new underground stations: ■ Domain(under St Kilda Rd in South Yarra)

■ CBD South (below Flinders Street Station) ■ CBD North(below Melbourne Central) ■ Parkville (under Grattan St and Royal Parade) ■ Arden (Queensberry Street) ■ Existing northern and Caulifeld rail groups will become four independent corridors, allowing more services on the Craigieburn, Sunbury, Werribee/Williamstown, Upifield, Frankston and Sandringham lines, and longer trains on the Sunbury, Pakenham and Cranbourne lines.

■ It would mean an extra 24,000 passengers an hour across the network