The massive cost attached to the MetroLink project in Dublin is disproportionate and 'doesn't stack up', Duncan Stewart has said.
An Bord Pleanála hearings into the project are continuing with a focus will on traffic management plans, land and property acquisition along the route.
Under current plans the new fully segregated railway, most of which will be underground, will run between the Swords and Charlemont areas of the capital.
The route will also serve Dublin Airport - the first rail link to the country's largest airport - as well as major educational campuses.
The route for the Dublin MetroLink. Image: MetroLinkThe overall project is expected to cost between €9 billion and €12 billion and is aimed for completion in 2035.
Architect and environmentalist Duncan Stewart told Newstalk Breakfast he believes it is not worth the money.
"With a starting cost of €9.5 billion we don't know where that's going to go in terms of cost," he said.
"That's an incredible amount of money that would be going to one exclusive project that would connect Dublin Airport - primarily it's about Dublin Airport.
"The cost of doing that, knowing that that money then will be deprived from all of the much more essential rail networks and Luas networks that we need to put into place that are critical to make Dublin a proper public transport network system.
"We need to be able to give people that choice that they can take public transport and know that it can be reliable and will get to where you want to get to.
"But the MetroLink does tick that box, it's very specific.
"It will be great for people going to the airport [who] want to go to from Dublin city but it's not going to be any good for people coming up from around the country or people across Dublin city.
"It does facilitate Swords... but really the massive cost, and we don't know what it will end up costing, is disproportionately out of line.
"It doesn't stack up".
Split-screen image shows an underground Metro train and an artist's impression of a MetroLink station in DublinMr Stewart said he is fully behind better public transport.
"I'm incredibly in support of public transport and we badly need an awful lot more public transport infrastureutre," he said.
"The problem we have is we've traffic congestion increasing as our population grows and as more and more people commute into Dublin.
"It's only going to get worse and worse unless we change and develop public transport.
"It's great to be underground, we all want to see trains underground but it's very, very expensive.
"The cost per kilometre of this is about €500 million".
Mr Stewart said if he was given €9 billion he has "20 projects" in his sights - ranging from an interchanging bus system to a DART line around the M50.
