Opening a passenger rail line under the Phoenix Park in Dublin would not be an adequate alternative to the construction of Dart Underground, Iarnród Éireann has said.

The National Transport Authority (NTA) recently announced plans to start passenger services to connect the city’s two main stations through the 136-year-old tunnel line between Connolly and Heuston. The journey would take about eight minutes.

The line, which passes under the park in a 692-metre-long tunnel is used to transfer engines and out-of-service trains, as well as some freight between the two stations. Until the opening of the Luas Red line it was used occasionally on busy Croke Park match days to transfer passengers from Heuston to Connolly. But apart from a brief period in the early 1900s, it has never been used for regular passenger services.

In its infrastructure implementation plan for 2013-2018, the NTA said that following the completion of engineering and signalling works it was intended to open the line to passenger services. These services could be running as early as 2015, it said. The line is intended to take passengers from commuter areas of Kildare, Carlow and Laois, who must transfer onto a Luas or bus to reach the city, straight to Connolly. But if the line were opened, passengers could go all the way from Cork to Belfast on one train.

The NTA’s Phoenix Park plan follows the Government decision in November 2011 to shelve plans for the €4 billion Dart Underground line, which would have connected the intercity and commuter lines with underground stations at the docklands, Pearse Station, St Stephen’s Green, Christchurch and Heuston.

However Irish Rail said the tunnel line was not an acceptable substitute for the Dart Underground as it would send extra trains into the Connolly Station bottleneck. “Only 12 trains can get in and out in an hour and, at peak times, we are at full capacity at Connolly, that’s why the docklands station was built,” a spokeswoman said.

The company is beginning a major resignalling project, which will bring the capacity from 12 to 20 trains per hour, but that would not facilitate anywhere near the capacity for the tunnel line as would be created by the Dart Underground, which would use the docklands station to bypass Connolly.

“The Dart Underground offered not just more capacity, but an awful lot more city centre connectivity. The tunnel doesn’t go through the city centre and really isn’t a long-term solution.”

Although the NTA draft plan states it is “intended” to go ahead with Phoenix Park passenger line, Irish Rail said there would have to be a detailed analysis before any decision could be made.

“We are only at the stage of initial consultations. There will have to be a cost-benefit analysis. There would have to be discussions with the communities along the line to see if there was a population to sustain it, and we would have to ensure that existing services would not be adversely effected,” it said.

Currently the only station along the tunnel line is Drumcondra.

Travelling from Connolly, the line takes the route of the Sligo line passing by Croke Park and Drumcondra Station. It parts company with the Sligo line at Glasnevin junction near the cemetery and heads south through Cabra.

It goes underground at the Garda headquarters in the Phoenix Park and exits the park under Conyngham Road before crossing the Liffey on a bridge that leads into Heuston.