Brendan Smith, speaking on behalf of transport minister Noel Dempsey, told the Seanad last week that the government would not reveal details of the project's cost as it would be "commercially sensitive".

The Metro North project was announced under the government's Transport 21 plan in late 2005 and is scheduled for completion in 2013. Using underground, surface and elevated tracks, it will go from St Stephen's Green to Swords, making 15 stops along the way.

Paschal Donohue, Fine Gael's Seanad spokesman, said this weekend: "We need excellent public transport and clear public transparency at the same time. That the government is planning the costbenefit analysis after the commencement of the planning process is disturbing."

An independent study which Donohue commissioned found that every trip on the proposed line would cost taxpayers at least 22, more than five times the 4 cost to taxpayers of every trip on the city's two Luas lines.