Originally the metro was seen as a way to ease chronic overcrowding on western suburbs CityRail services by utilising the mostly empty country platforms at Central for these trains, instead of having them continue through the city and over the Harbour Bridge. Under the plan, at least 26 trains an hour in the peak would shift to the country tracks and terminate at Central. Passengers would be forced to change to empty metro carriages at a new station beneath Central's western forecourt. But senior government officials have realised it is impossible to terminate so many trains hourly at Central because there is no way to get a sufficient number of trains back out of the station in time. Sources have told the Herald that this plan to change the CityRail network, originally submitted to Infrastructure Australia, has been withdrawn and is being revised. Alec Brown, a spokesman for the Sydney Metro Authority, said: "It is normal in these kinds of circumstances for a proposal to be altered or revised during ongoing discussions between agencies, to get the best outcome for commuters. The metro system and heavy rail will be compatible at the opening of the CBD Metro in 2015."

But if these western line trains do not terminate at Central and continue on to the North Shore, as they do now, via Town Hall and Wynyard, so few people are likely to use the CBD Metro that it will be an outright failure. In that scenario, there will be no reason for passengers to change to the metro because their trains already travel to their final destination stations in the northern parts of the city. At Central, the first problem is that three of the 15 platforms have to be quarantined for country trains, which are too long to terminate anywhere else on the network. For 26 trains an hour to use the other 12 platforms, at least 14 trains an hour must move out of their way. And because there is just a single pair of tracks returning west, these 14 trains have to cross the flow of the 26 trains entering the yard - a task which RailCorp bureaucrats have acknowledged is unattainable. "It is virtually impossible to manage that level of frequency at a terminal station," one insider said. The plan faces other huge challenges. A big railway flyover at Blacktown would have to be remodelled at enormous cost to allow Richmond line trains access to the country tracks that terminate at Central. Similarly, a vital track junction at Strathfield would have to be overhauled, which could have severe effects on Strathfield station.

And new automated signalling systems, not yet in use anywhere in the CityRail system, would be required between Strathfield and Chatswood and between Parramatta and Central Station - almost a quarter of Sydney's entire network. Experts put the cost of this resignalling at more than $2 billion and say the concept is unproved on the scale required in Sydney. OFF THE TRACKS * Metro is announced in October before robust feasibility analysis. Within weeks the cost balloons by $800m. * The Ministry of Transport warns in December of "potentially competing objectives" between the metro and an upgrade of Victoria Road.

* RailCorp warns the Department of Planning in January that the metro's proposed route would block a vital CityRail extension. * Infrastructure Australia board advises the metro does not merit federal funds. * Pyrmont light rail company demands compensation of up to 50 per cent of its revenue should the metro go ahead.

