Four passenger trains an hour will use the Phoenix Park tunnel at peak times when it reopens next year, the Herald can reveal.
Heuston station is currently the end of the line for commuters from Kildare, Portlaoise and farther afield, with Luas and bus services taking them the last 3km into town.
Once back in service, the tunnel will allow trains to deliver passengers into the heart of the city, as well as connecting them to the Dart.
The re-opening of the 1877 tunnel was announced last year as part of a €900m transport plan for the capital.
A letter from the National Transport Authority (NTA) to South Dublin County Council last month said the planned train services will operate from the spring or summer of 2016.
The service will begin following the completion of the city centre re-signalling project which will increase capacity through what is described as a "critical city-centre section" between Connolly and Grand Canal Dock stations.
"It is anticipated that initially, there will be four services per hour from Kildare/Portlaoise through the Phoenix Park tunnel, terminating in Grand Canal Dock in peak hours, with an hourly off-peak service," the letter said.
Commuters boarding trains for the capital at Portlaoise, Portarlington, Monastarevin, Kildare and Newbridge stations will benefit from the re-opening of the 692-metre tunnel.
The Rail Users Ireland lobby group gave a guarded welcome to the news.
"If they're adding extra trains everyone is happy," said spokesman Mark Gleeson.
However, he warned that an "awkward situation" will arise "where some people will benefit and others won't" if existing trains are diverted for the new service.
Most traffic through the tunnel is carriages and engines being moved between Heuston and Connolly for maintenance.
It has occasionally been used for special services, including major GAA fixtures, but less so in recent years.
The line emerges from the tunnel parallel to the North Circular Road and passes through Cabra and Glasnevin, connecting with the track that goes through Drumcondra station and on to Connolly.
Along with the resignalling, an NTA spokeswoman outlined work needed to bring the route into service.
"There are various items of repair work required, along with changes to Drumcondra station and the introduction of additional ticketing gates. The approximate cost of these works is about €10m," she said.
The proposal to re-open the tunnel was incorporated into the €900m Integrated Implementation Plan, for 2013-2018.
Former Transport Minister Leo Varadkar took a special interest in the project when he was appointed in 2011.
"One of the first things I did in office was to travel through the Phoenix Park tunnel with my staff to examine it first hand, as the public authorities had been rejecting this proposal for years," he said at the time when the plan was announced.
Mr Varadkar said it was the re-signalling works that were the "game-changing" component that would allow the tunnel to reopen.
The Integrated Implementation Plan sets out the reasoning behind the tunnel's use.
"Under the current configuration, rail services entering Dublin city on the Kildare line terminate in Heuston Station," the plan states.
"These services include a mix of inter-city trains from Cork, Waterford, Limerick and Galway as well as commuter services from Kildare, Carlow, Newbridge and Portlaoise.
"Heuston station lies some 3km from the commercial core of the city and in excess of 3km from the area of highest density employment in the south eastern quadrant of the city."
The report points out that commuters entering the city through Heuston currently have to use Luas and bus services to get to the city centre.
Opening the tunnel will give them a direct route to offices and shopping areas in the he art of the city.