Iarnród Éireann revealed its plan for a 5km rail tunnel linking Heuston Station with Dublin's docklands via St Stephen's Green and Pearse St yesterday.
However, it warned that the project would cost significantly more than the €1.3 billion estimate put forward when the scheme was first proposed in 2002.
The new underground interconnector is a separate project from the metro between St Stephen's Green and Swords, which is planned by the Railway Procurement Agency.
Iarnród Éireann was urged yesterday to rethink its project in order to prevent Dublin ending up with two underground railways which would could not be connected. Company spokesman Barry Kenny confirmed that the gauge of the underground rail interconnector would be different to the metro but he said passengers would be able to change services at a number of points including St Stephen's Green and the docklands.
Plans for the Iarnród Éireann interconnector are to go on display at the Dublin Civic Offices in Wood Quay next Tuesday and Wednesday, during which the public will be asked for opinions on the alignment and station locations.
The company said the line, described as a high capacity underground Dart line, "will complete the transformation of the Greater Dublin area's rail service capacity from 33 million passenger journeys annually now to over 100 million passenger journeys".
Mr Kenny said that while the final costs of the project were not yet evaluated, they would be "well within" the overall provision contained in the Government strategy Transport 21.
Transport spokesman for the Progressive Democrats Senator Tom Morrissey urged Iarnród Éireann to rethink the design of the tunnel to prevent the creation of two underground railways in the city which would not be open to each other's trains.
