The extension of the Luas green line a further 8 kilometres in south Dublin to Cherrywood is needed now but will have to be replaced by a Metro in 2020, a report revealed today.
Following months of public consultation the Railway Procurement Agency was given the go-ahead for 11 new stops on the route but the Government was warned it would soon need upgraded.
The line could take over three years to complete and in another 10 years longer trains would be badly needed to meet demand.
The report of the public inquiry stated: "Overall the public reaction to the proposed extension of the Luas B1 Line from Sandyford to Cherrywood has been positive, and generally financial and economic evaluations of the project indicate a positive case for introduction of the project.
"The inquiry is satisfied that there is a need for the Luas Line B1 extension as proposed and that the project is viable."
And it went on: "The inquiry is satisfied on the information presented by the RPA that the Luas line extension has the capacity to meet users' needs until 2020 after which upgrading to Metro will be necessary."
Eamon Ryan, Green Party transport spokesman, said the report makes an unanswerable case for the Airport Metro to be connected to the south side of the city from day one.
"Metro trams would be twice the length of existing Luas trams, would have a far more frequent service and would therefore provide a much larger carrying capacity," he said.
The Greens made a submission to the rail inspector that the new line should be built to Metro standards and insisted that both Luas and Metro trams will run on the same line.
Mr Ryan said: "It now makes no sense for the Government not to extend the Metro service to the southside at the same time that they are building the Metro line from Swords to St Stephen's Green."
He said the Metro should run from the airport underneath the city centre to St Stephen's Green and on to the Beechwood stop on the Luas green line near Ranelagh where overground services would take over.
"If we have learnt one thing about transport planning in this city over recent years, it should be to provide the proper capacity from the start rather than trying to cut corners and then come back later," he added.
A total of 11 stops are planned along the 7.6 kilometre route.
Starting at Sandyford industrial estate it will run alongside Blackthorn Avenue towards Ballyogan Wood, Leopardstown racecourse, Brennanstown Vale, where a 350-space park and ride facility will be built, and on to Laughanstown Lane before ending at Cherrywood.
All trams on the green line will initially be 40m long, carrying 310 people each, but can be extended to 50m. Journey time from Cherrywood to Sandyford is about 19 minutes, giving a total journey time on the route to St Stephen's Green of 41 minutes. Construction on the line is expected to take over three years.
And the report said road traffic inconvenience from Luas works will be within acceptable limits. The report also stated there had been adequate public consultation in advance of the application to extend the green line. The report said the two existing Luas lines carried between them over 22 million passengers in 2005.
Around 32,000 people use the green line on a daily basis while the weekly average number of users is over 220,000 people. It revealed five pedestrians were injured on the tram lines and the accident ratio for 2005 is 0.16 accidents per 10,000 tram kilometres travelled — well below that for other comparable European cities.