AN EXTENSION of the Dart line to Dublin Airport is not a realistic alternative to Metro North, according to the National Transport Authority (NTA).
A Dart airport link would only address a "fraction of the public transport needs" between Swords and Dublin city centre, the NTA's Hugh Creegan has said.
His comments come as Transport Minister Leo Varadkar prepares to announce whether plans for Metro North will be shelved.
Speculation had grown that the he would opt for a Dart spur as a cheaper alternative to the underground rail link.
But Mr Creegan's appraisal makes clear that the more expensive, higher frequency model would be preferable from the NTA's point of view.
He said Irish Rail has made a "preliminary business case" to the NTA in favour of a Dart link from the airport to Clongriffin station on the northside.
In a letter to Fingal County Council, he pointed out a draft transport strategy identified that the north-south corridor between the city centre and Swords has "a high level of public transport demand, which is projected to increase significantly in future years".
"Metro North has been planned and developed in order to deliver a high capacity public transport mode that services the forecast demand," the NTA director stated.
Mr Creegan added: "It also links the airport with the Swords area, home to a large share of airport workers."
Some 20pc of Metro North trips are projected to be taken by airport staff and passengers.
The other 80pc would come from the other locations served, including Dublin City University, the Mater and Rotunda hospitals and Croke Park.
"Implementing the Dart airport link provides a public transport rail service to Dublin Airport only. It does not serve the other needs along this Swords to city centre corridor," Mr Creegan said.
Figures show that Metro North would carry at least 14 times more passengers during it peak hour than the Dart.