Transport chiefs have refused to put a cost on the MetroLink project, admitting they cannot provide one "with a high degree of accuracy".
The light railway was originally planned to have two tunnels, stretching 26km from Swords in north Co Dublin to Sandyford in the south, and cost €3bn.
However, a new "preferred route" has seen a major curtailing of the project, which will now have only one 19km tunnel, ending at Charlemont in the city centre.
Asked if the cost will at least be under €3bn, National Transport Authority (NTA) deputy chief executive Hugh Creegan said it was "simply too early to put a firm and realistic cost in place".
MetroLink project director Aidan Foley said the plans had changed significantly since last year.
"We can't provide a cost forecast with a high degree of accuracy," he said.
Other major changes to the proposed MetroLink route include:
It will take between six and seven years to build with trains due to start running in 2027.
Under the new route, the number of homes that will be bought through a compulsory purchase order has fallen from 105 to 85.
These include 70 apartments at the College Gate complex and eight rented properties owned by Dublin City Council, all on Townsend Street.
They, along with the Markievicz Leisure Centre, will be demolished to make way for a station.
Gordon Rose, of the Save College Gate Campaign, said residents were "pretty devastated" to learn that their building is still in line for demolition.
The NTA said it will help tenants to find alternative accommodation and pay their rent for up to one year.
In the case of owner-occupiers, they will be paid "appropriate compensation" and be assisted in finding a new home.
The NTA said it is consulting with Dublin City Council about building a replacement leisure centre.