The National Transport Agency (NTA) has backed the decision to delay the DART Underground, saying the reasons for building it have changed in the meantime.

The Dáil's transport committee has been told that the original plans for the line no longer apply, because of other improvements made since it was first announced.

Rail chiefs are examining three alternatives to the abandoned €3bn plan for a Dart Underground to link intercity and Dart lines beneath Dublin.

The Government confirmed in September it was shelving the original design for twin 7.5km tunnels, 25m below Dublin, to link railways from the west and south to the northern lines.

But it will be another four years at least before construction begins.

The National Transport Authority told the Joint Committee on Transport and Communications that rail planners were looking at an option of boring a tunnel from the Docklands to Heuston station rather than the original idea of boring all the way to Inchicore.

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Anne Graham, the NTA chief executive, said a second possibility is to go underground between Heuston and Pearse Street – avoiding the expense of a tunnel under the Liffey.

The third proposal is to bore the tunnel only on the southern side of the Liffey but with fewer intermediate stations.

Pearse Street was intended to be the main passenger hub for Dart Underground with the commuter rail service planned to operate from Greystones in the south, to Drogheda in the north and Maynooth in the west.

Ms Graham said the original Dart Underground was “a very significant cost”, not “economically justifiable” and a cheaper version was possible.

The shelved plan envisaged new lines running from north of Connolly Station into Spencer Dock in the Docklands before dropping under the Liffey to Pearse Street and subsequent stations at St Stephen’s Green and Christchurch before heading west to Heuston and Inchicore.

It has been claimed €40m has been spent on the project already.

Dart Underground was to cost €3bn with another €1bn estimated for works on the Dart Expansion programme which included electrifying northern railway lines to Drogheda, again on the Cork line between Heuston and Hazelhatch and again from Connolly to Maynooth on the Sligo line.

It also involved creating four tracks from Parkwest to Inchicore and removing level crossings, resignalling and investment in new fleet and depot facilities.