Iarnród Éireann has warned that its network will become clogged within eight years unless a €1.3 billion tunnel linking Connolly and Heuston Stations in Dublin is built.
In a submission to the Government in July, the rail company said that the current network was due to reach capacity by 2012 and would be unable to cater for projected increases in population along its routes.
In an ambitious €3.4 billion plan contained in the submission, the company said it planned to be able to provide double-decker trains which would run on a commuter line from Drogheda, through the tunnel and on to Kildare. The interconnector tunnel would have five underground stations through the city, serving key areas such as St Stephen's Green and Pearse station.
The company has informed the Government that it could have the Connolly-Heuston tunnel built by 2011.
Other key projects in the proposal include extending the DART to Maynooth and Dunboyne, along with a further DART spur off the existing Howth line to the airport.
Iarnród Éireann has also proposed a €90 million investment for a commuter rail development in Cork.
Under the current €2 billion investment plan for the railways, including extra trains, tracks and new signalling, the company will be able to increase its passenger numbers from 35 million last year to nearly 45 million by 2008.
A company spokesman said that after 2008, it would be unable to keep up with increased demand from extra commuters unless the interconnector was built.
Figures collated by the company suggested that population along commuter lines serving Dublin will increase by 300,000 between 2002 and 2015.
Describing the project as "a necessity" rather than a possible alternative to a metro system, he said without it, demand would begin to outstrip any increased capacity from that date.
"People are coming and we know where they are coming to these routes," he said. "The main problem in the current network is the fact that the two main stations, Connolly and Heuston, do not link up." This was exacerbated by the fact that the only viable passenger link across the Liffey, the Loop Bridge, was restricted to fewer than 10 trains an hour, which severely limited capacity on the entire network.
The interconnector would remove Drogheda and north-bound intercity trains from the bridge, enabling DART trains to travel from Maynooth to Greystones.
Iarnród Éireann's plans are among a series of public transport proposals which are being being considered by the Government. They include plans for a metro and proposals to create a series of new Luas lines.
A spokesman for the Minister for Transport, Mr Cullen, yesterday said that a decision on the interconnector tunnel would be made as part of a wider Government plan for public transport investment.