Work has begun on a new underground DART tunnel that Iarnród Éireann says will quadruple the number of passengers on its Dublin network.

The company has published a proposed route for the Interconnector line that will run from the docklands to Heuston Station stopping at Pearse Street, St Stephen's Green and Christchurch.

Iarnród Éireann is already building underground facilities at Pearse St, although the Interconnector's first passengers are not expected for seven years.

But the company says this underground DART project is the most important in the State in terms of encouraging a switch to public transport.

A company spokesman said the line would quadruple Dublin rail passenger journeys to 100m a year.

This is because the line will increase capacity and frequency by linking all the rail services on DART, LUAS, Commuter, Intercity and Metro.

For instance the interconnector will link with the Metro North at St Stephen's Green.

The Christchurch station caused some problems because of its sensitive location, but Irish Rail has decided to locate a subway station near the Civic Offices, which is an area already excavated during the Wood Quay controversy.

This station will also have an entrance from Temple Bar and similarly the St Stephen's Green stop will also be accessed from Kildare St and the Pearse St station will be accessible from Merrion Square.

Iarnród Éireann is applying for a railway order next year, the equivalent of planning permission, and hopes to start tunnel boring by 2011.

The first passengers are expected in 2015.