The expansion of the DART originally included an 8.6km tunnel from Inchicore to East Wall with stations at Inchicore, Heuston, Christchurch, St Stephen’s Green, Pearse Street and Spencer Dock.

Rail bosses are developing a scaled-down version of the Dart Underground expansion project in an attempt to secure Government approval to allow construction work to begin in just over three years' time.

Shorter tunnels, which would require passengers to change trains, are among the options being considered, as the company attempts to reduce the massive €4.5bn bill.

The expansion of the Dart originally included an 8.6km tunnel from Inchicore to East Wall with stations at Inchicore, Heuston, Christchurch, St Stephen's Green, Pearse Street and Spencer Dock.

Dart trains would then continue to Clontarf Road Station, and then to Howth/Malahide and Drogheda.

When complete, it would result in the city having two Dart lines - one running from Maynooth to Greystones of almost 65km, and a second from Drogheda/Howth to Hazelhatch in Kildare of 76km.

But the project was shelved in 2015 amid concern that passenger numbers would not materialise.

The total cost of the scheme is some €4.5bn - of this, the tunnel makes up €2.65bn under its current design. The remainder includes the cost of resignalling, new fleet and other measures.

Irish Rail engineers are now exploring options for a scaled-down version.

Three alternatives are being assessed. They are:

Shortening the overall tunnel length by removing the section between Inchicore and Heuston. Instead, additional running lines would be built above ground. This could involve the removal of a station at Inchicore.

Building the tunnel but terminating it at Pearse Station where it would connect with the existing Dart system. This would reduce capacity.

Build the connecting lines, but passengers would be required to change trains at Heuston to connect with the Kildare line and Hazelhatch.

"A full assessment is to take place, and we are assessing all options," an Irish Rail spokesman said. "We are looking at those redesign options, will look at the cost-benefits of each and will be providing that to the National Transport Authority.

"We could then proceed to a railway order (planning permission), with construction works to start in 2020."

Currently, some 20,000 cars and taxis enter Dublin city centre in each hour of the morning peak, which is resulting in serious congestion on roads into the capital.

Traffic volumes on the M50 have increased by 6,300 a day in the last year, leading to major tailbacks.

Transport experts say additional public transport services are required to remove cars from the road.

Among the issues to be addressed under scaled-back plans include whether cost savings would have a severe impact on the level of service to be delivered. The review will be completed by the end of 2017.

Currently, Irish Rail has capacity to bring 43,700 passengers into the city every hour - 7,600 from the Kildare line which includes trains from Cork, Galway, Limerick, Waterford, Tralee and Westport; 4,500 on the Sligo/Maynooth line; 17,100 from the northern line serving Belfast, Drogheda and Dart services; and 14,500 on the south-east line from Rosslare which also hosts the Dart.

But Dart Underground would provide increased capacity of 73,300 per hour - an additional 30,000. Kildare services would increase to 18,800; Sligo/Maynooth to 15,150; the northern line to 24,200; and south-east line to 15,150.

The main interchange would be at Pearse Station, and the expanded network would interchange with Luas and bus services.

"Dart expansion has the potential to eliminate all peak hour (car) trips into the city centre," Irish Rail documents state. The scheme would take seven years to build.

Irish Independent