TWENTY ELECTRIC trains per hour - one every three minutes - would run through the planned tunnel linking Heuston Station with Docklands, according to Iarnród Éireann.
But the tunnel is intended not only to provide the missing link between Dart and suburban commuter rail services in Dublin; it will also transform services on the existing Dart line.
Dart trains originating in Bray or Greystones would terminate at Maynooth, while those originating in Howth or Malahide would run through the tunnel to Hazelhatch and, ultimately, Kildare.
Anyone wishing to travel from, say, Dún Laoghaire to Howth (or vice versa) would have to change at Pearse Station, Westland Row; the continuous service around Dublin Bay would cease.
One major benefit, according to Iarnród Éireann, is that the current bottleneck at Connolly Station would be freed up, enabling the company to provide a wider range of rail services.
Another major benefit - indeed, the raison d'etre for this €2 billion project - would be to bring an end to the historic isolation of Heuston Station, knitting it into a new suburban rail network.
The journey time between Heuston and Docklands would be less than 10 minutes, with just three intermediate stops - Christ Church, St Stephen's Green and Pearse Station, all roughly 1km apart.
Along the way, the Dart underground - as the company has branded it - would link up with the Tallaght Luas line at Heuston and Docklands as well as the Sandyford Luas line at St Stephen's Green.
"Every rail line into Dublin will see a massive increase in capacity from this project," according to Iarnród Éireann spokesman Barry Kenny. "Without it, we would have guaranteed gridlock for evermore."
Resignalling at Connolly is intended to increase its throughput from 12 to 18 trains per hour in each direction, including the Maynooth line, but even this would be less than the interconnector's capacity.
The company forecasts that the number of peak-hour passengers using its commuter rail services in the Greater Dublin Area would increase almost threefold, from 22,000 now to 62,000 in 2020.
Mr Kenny said four-tracking the Kildare line as far as Hazelhatch - currently its biggest rail project, costing €350 million - would be "dwarfed" by the investment in the Heuston-Docklands tunnel.
Electrification of the Kildare line to Hazelhatch is provided for in the Government's Transport 21 programme. The Maynooth line would also have to be electrified in order to make Dart Underground work.
Diesel engines would only be used in the tunnel section after hours for maintenance; otherwise all trains would be electric. Anyone travelling to or from Kildare would have to change at Hazelhatch or Heuston. Asked why no station was being provided to serve the Digital Hub in Thomas Street, project manager Peter Muldoon said this area was within walking distance of Heuston or the proposed station at Christ Church.
It is also intended to retain the relatively new station above ground in Docklands, probably as a terminus for the proposed Navan commuter line. This would be 100 metres from the entrance to the underground.
The Irish Times