The proposed southside leg of the MetroLink line is to be scrapped when the preferred route is announced by transport chiefs today.
The line will not go all the way to Sandyford as had initially been mooted.
That route had sparked opposition from residents and fears of years of disruption to the Luas Green Line.
The Herald has learned key details of the new plans to be unveiled by the National Transport Authority (NTA) and Transport Infrastructure Ireland (TII).
They include the Metro running from Swords to Charlemont Street to meet the existing Luas service.
Trains will not continue down the Green Line to Sandyford as had been proposed.
Drilling is to continue as far as Ranelagh to allow expansion in future years.
Green Line Luas services are to be upgraded over the next decade to the point where there should be a 55-metre tram every two minutes to deal with increasing demand.
A period of public consultation is to be opened up until mid-May before the preferred route proposals move to the planning stage.
Officials have identified 2038 as a possible date for expansion farther south from Charlemont.
"By that stage it's hoped further development would be less disruptive to commuters," a source said.
The NTA will not reveal any costs at this stage as the project is going to go back to public consultation.
An international expert, Prof Bent Flyvbjerg, from Denmark, is to be brought in to oversee the costings.
He deals in the management of mega-projects and has worked on the Olympic Games.
The original proposed route sparked controversy on both sides of the city when it was announced last year.
There were complaints from Ranelagh residents about the closure of a road from Dunville Avenue to Beechwood Road during construction.
The original plan would have seen the MetroLink go underground as far as Charlemont.
It would then have continued overground along an upgraded Luas Green Line.
That led to claims it could create a "Berlin Wall" dividing the community during construction.
Another plan for an underground route through Ranelagh could have seen the Luas line close for up to four years.
This was ruled out by Transport Minister Shane Ross as "completely and utterly un- acceptable".
On the northside, there was an outcry from members of Na Fianna GAA club when the original plan would have seen them lose playing pitches for up to six years during construction.
Its pitches will not be affected in the new plan, nor will a planned €3m facility for the Ballymun Kickhams club near Dublin Airport.
Local TD Noel Rock said plans for the facility are not in jeopardy as the site is large enough to accommodate both the new pitches and the Metro- Link tunnel boring machines' entry location.
Mr Rock said the proposed MetroLink plans can be considered a "definite victory" for both the Ballymun Kickhams and Na Fianna GAA clubs.