A proposed underground rail line beneath Dublin city centre would link Spencer Dock with Pearse Station (Westland Row), looping around to serve St Stephen's Green, Dame Street and the Civic Offices at Wood Quay and then running on beneath the south quays to Heuston Station.
Given that CIE has already proposed that Dublin Airport should be served by heavy rail, branching off from the Sligo line and terminating at Spencer Dock, the latest proposal would make it possible for people to get a train to the airport from key locations in the city centre.
The proposed underground is being considered in the context of a strategic study of rail services in Dublin commissioned by CIE from international engineering consultants Ove Arup and Partners. The study is due to be completed before the end of this year.
The consultants are examining all suburban rail options, including a link to Dublin Airport, reopening the line serving Navan, and providing extra tracks on the Kildare line, as well as the thorny issue of a strategic cross-river rail link downstream from the Loop Line bridge over the Liffey.
Until now consideration of a new cross-river rail link, endorsed by the Strategic Planning Guidelines for the Greater Dublin Area, was confined to linking Spencer Dock with a new DART station planned for Barrow Street, in the Grand Canal Docks area south of the Liffey.
Because this link is regarded as being of such strategic importance, Dublin Corporation's planning decision in August on the massive development scheme proposed for Spencer Dock, including the National Conference Centre, was predicated on resolving it.
The corporation's decision specified that development of much of the Spencer Dock site "should not be allowed to proceed pending a decision by the competent State authorities in relation to the vertical and horizontal alignment of the proposed cross-river rail link".
The corporation made it clear that its planners favoured an underground link, primarily to avoid having to erect a possibly unsightly rail bridge in close proximity to the dramatic suspension bridge proposed to spring across the Liffey from Macken Street.
This bridge, which has been designed by the Catalan architect-engineer, Dr Santiago Calatrava, would carry road and pedestrian traffic as well as a possible Luas light rail line serving the Docklands area. It would swivel to accommodate river traffic.
Given that CIE, as freehold owner of the 51-acre Spencer Dock site, currently used for rail freight, is anxious to preserve as much as possible of its ground space for development, the scheme now under appeal to An Bord Pleanala provides for an underground station.
Technically, it was always going to be difficult to link such a station with Barrow Street, given that the latter is elevated some two storeys above street level. An overground link would also have necessitated expensive property acquisition, as well as causing serious severance.
The option now being seriously examined would involve taking the rail link underground to Pearse Station, Westland Row, where there would be an interchange with the existing DART line, and then on through the city centre via St Stephen's Green, Dame Street and the south quays.
And because it would continue to Heuston, the underground line would end its relative isolation from the rest of the city's rail-based public transport services.
It is also planned to upgrade the line through the Phoenix Park tunnel, linking Heuston and Connolly stations.
Nearly a quarter of a century has passed since the Dublin Rail Rapid Transit Study first recommended a city-centre underground, linking Connolly Station to Heuston, with commuter railway lines extending outwards to serve Tallaght, Clondalkin and Blanchardstown.
But this plan, together with its proposed central station in Temple Bar, was dropped by the then Fianna Fail government in 1987, largely on grounds of cost. Instead, CIE was told to concentrate on bus-based and diesel rail options to cater for public transport in Dublin.
Subsequently, in its 1994 final report, the Government-sponsored Dublin Transportation Initiative recommended that light rail should be added to the equation, and this led to the formulation of plans for a three-branch Luas network serving Tallaght, Sandyford and Ballymun.
Lobbying by the business community, notably Dublin Chamber of Commerce, against plans to run Luas on-street through the city centre led the present Government to decide in May 1998 that light rail should go underground between St Stephen's Green and Broadstone.
Critics pointed out at the time that this expensive option - now being examined in detail by British consultants W.S. Atkins, who had endorsed the on-street proposal - would do nothing to increase the capacity of Luas to carry more passengers between Sandyford and Ballymun.
E now seems to have accepted that argument. Under the scheme now being considered, a city-centre underground would consist of a pair of bored tunnels, requiring minimal disturbance at surface level. Their diameter, at 15 feet, would be marginally more than the Grand Canal drainage tunnel installed in the 1970s.
The scheme is not seen as a substitute for Luas, but rather as a complementary facility. Thus passengers on the Luas line from Sandyford would be able to change to the underground in St Stephen's Green, while those on the Tallaght line could change at Heuston.
The installation of a metro-style city-centre underground line would obviate any need to put the Sandyford-Ballymun Luas line underground at St Stephen's Green.
Detailed site investigations being carried out by W.S. Atkins would not be wasted; their data could be used to flesh out the latest underground proposal.
Pearse Station at Westland Row would clearly become a major hub as the principal interchange between DART and the underground line; this would also be in CIE's interest as it is planning a major development centred on this station in partnership with Trinity College.
The cost estimate of £500 million is not seen as an impediment, given this year's Exchequer surplus of £5,000 millionplus, and CIE is expected to press strongly for the inclusion of the proposed underground in the forthcoming National Development Plan.
The project could also be a candidate for a public-private partnership deal, in line with the Government's intention to raise additional funds from the private sector to finance public infrastructure, assuming that the required legislation and procedures can be put in place.