Dublin's Luas system, due to commence operations in nine weeks time, needs to carry a similar amount of passengers as the city's DART system just to cover its operational costs, an economist has claimed.
Luas expects to carry 20 million passengers in its first year of operations at an expected "fare box" revenue of €20 million.
It has also said it expects to pay the international transport operator Connex €20 million a year to run the system.
However, a leading economist, Mr Colm McCarthy, of DKM Consultants, has cast doubt on the viability of the system based on its own projections, pointing out that projected passenger numbers for the two unconnected lines are similar to those for the entire DART system.
Iarnród Éireann has confirmed its DART service between Howth, Malahide, Portmarnock and Greystones carried 21.6 million passengers in 2003.
According to Mr McCarthy, the Luas lines are extremely unlikely to meet their target of 20 million passengers in their first year of operation.
If the target is not met, Luas will potentially be unable to meet its financial obligation to pay Connex its fee of €20 million.
The figures also cast doubt on the financial viability of other extensions of the Luas to Cherrywood and the Docklands, for which private sector companies in the vicinity of the lines have been required to contribute.
Commenting after the British National Audit Office reported new tram lines across the UK had failed to achieve predicted passenger numbers - sometimes by 45 per cent - Mr McCarthy said difficulties with such systems were now "a worldwide phenomena".
Speaking to The Irish Times, Mr McCarthy said the real equation "is that trams generally cost twice as much, take twice as long and carry half the numbers, and this has been borne out by experience right across the world".
Asked if there was any reason to imagine that the Republic could be more accurate in its projections, Mr McCarthy insisted that Luas was already over-budget and over-time.
"Two legs of the treble are already up. I would love to be able to say that this is only 'mickology' but it is a worldwide thing."
However, Mr Ger Hannon, from the Light Rail Procurement Agency, said the agency was tired of hearing that Luas was over-budget and late.
He insisted that the system's passenger targets were to the highest international standards, and added that the Government had accepted that from the moment the contract was signed to the present, Luas was on budget.
As regards time, he conceded that the system may be about eight weeks off schedule.
Mr Hannon also insisted that the agency's estimates for passenger numbers were conservative, with eight million or nine million on line B, the Green Line to Sandyford; and 12 or 13 million on the Tallaght line in the first year of operation.
