Everyone seems to agree that the decision to be made today on the £263 million Luas light rail project is the most important this Government will take on the future of public transport in Dublin because of its profound implications for the capital.
The Cabinet is believed to be split, with Progressive Democrat Ministers arguing in favour of an underground option as the best long-term solution. Some Fianna Fail Ministers are also concerned about the disruption Luas would create during its two-year construction phase.
Despite the recommendation by independent consultants, W.S. Atkins, that the on-street project proposed by CIE should proceed, powerful vested interests have continued to argue that Luas should be put underground in the city centre to avoid disruption.
What primarily worries IBEC and Dublin Chamber of Commerce is not the short-term disruption caused while the system is under construction; they are opposed to an on-street Luas because they cannot conceive of city centre road space being "sacrificed" for public transport.
Yet the latest survey by the Environmental Protection Agency shows that air pollution caused by traffic in central Dublin, notably at College Green, now exceeds tighter limits proposed by the EU Commission and this is likely to cause increased levels of cancer and respiratory diseases.
The Tanaiste, Ms Harney, won well-deserved plaudits as Minister of State for Environmental Protection between 1989 and 1992 for working to eliminate the smog which once enveloped the city. Now, however, Ms Harney has apparently become the champion in Cabinet of putting Luas underground in the city centre - a prescription which would do nothing about the traffic causing high levels of nitrous-oxide and other air pollution.
With an underground solution there would be no compelling need for Dublin Corporation to introduce a radically different traffic management regime in the city centre, primarily to provide an "ideal operating environment" for an on-street Luas system.
Without such a regime, under which traffic would be diverted away from the central area, the Atkins report makes it clear that the underground option would "bring about a significant deterioration in the inner city environment", compared to the on-street alternative.
The costing of the underground option by Atkins, at an additional £240 million, is also tentative; it could add as much as £400 million to the bill for Luas, as the report suggests. Much would depend on geological tests and on the conditions found after tunnelling got under way.
Atkins also found that the earliest date on which a part-underground Luas could be completed would be 2005, compared to 2002 for the on-street project. This is because it would mean going back to the drawing board and producing a revised environmental impact statement.
And whereas CIE's current scheme has already been the subject of a public consultation process, the underground option has not. Thus, its negative impact on such sensitive areas as St Stephen's Green and the Iveagh Gardens, where trees would be lost, has yet to be teased out.
Mr Donal O Brolchain, of the Drumcondra 2005 group, said the extra cost of going underground could be used to finance the entire rail network proposed by the Dublin Transportation Initiative, serving Ballymun, Finglas, Cabinteely and, possibly, the airport.
It is understood that the CIE light rail project team is ready to begin public consultations next month on the line to Ballymun, which had been deferred when it became clear that there would not be sufficient EU funds to finance it under the current round.
However, the EU Commissioner for Regional Policy, Ms Monika Wulf-Mathies, has indicated that it would be prepared to assist in funding the design of a line to Ballymun and would "look favourably" on financing its implementation under the next tranche of EU structural funds.
The Atkins report cautions that any decision to run Luas underground would pose "a potential risk" to the funding of other elements of the DTI strategy because of the additional costs involved. "In effect, it would mean tearing up the DTI," said one source.
This source also stressed that there was "no possibility" of drawing down the £114 million in EU aid pledged for the Luas project if it went underground. "There would have to be contractual commitments in place by the end of 1999 to spend the money and that just can't be done."