Putting Luas underground between the canal ring and O'Connell Street would cost at least £500 million, roughly double the cost of the on-street system proposed by CIE, which the W.S. Atkins study prices at £268 million, or £41 million more than the current estimate.
It points out that there is "greater certainty" about the estimate for the surface option because the CIE scheme has already been developed. In dealing with the underground option, they say they took into account geo-technical considerations, but without drilling boreholes. The study also refers to the heavily canvassed "Unified Proposal", which would take the Tallaght line underground in Crumlin and the Dundrum line underground at Hatch Street. It puts the total cost of this scheme, including 7.2 kilometres of tunnel, at £547 million.
The "core tunnel" alternative examined in detail by the consultants would involve taking the Tallaght line in through Mount Argus, where it would go underground via Rathmines to Charlemont Street. There, it would join the Dundrum line, sharing a tunnel as far as O'Connell Street.
There would be underground stations at Hatch Street, St Stephen's Green and Trinity College and Tara Street, where it would connect with DART. The total length of tunnel would be reduced to 5.5 kilometres, with an overall capital cost of £500 million.
The "core tunnel" scheme was selected taking into account likely passenger demand, operating costs and fares revenues, ease of extending the network to other parts of the city, ease of interchange with other public transport, potential for avoiding traffic congestion and relative capital cost.
For the purposes of the comparative evaluation, the study assumed that the underground section would comprise two single-track tunnels, each 5.4 metres in diameter, constructed either by tunnel boring machine or by machine and hand excavation. Vehicles would be slightly wider than the proposed on-street trams.
The surface option reviewed by Atkins is the CIE scheme, under which the Tallaght-Dundrum line would run in via the Naas Road, Heuston Station, Smithfield and Abbey Street to O'Connell Street and then out via College Green, Dawson Street, St Stephen's Green and the old Harcourt Street railway line.
The total length is 22 kilometres, with 33 stops, and the service would be operated by 29 light rail vehicles, each 30 metres long and travelling at average speeds of 21 kilometres an hour. However, the consultants assumed that 40metre light rail vehicles would be used to cater for the level of passenger demand now being predicted.
The study puts the capital cost of building the full extended Luas system (serving Ballymun, Cabinteely, Docklands, Finglas, Swords and Tallaght) at around £850 million for the underground option and £515 million for the surface option, excluding the cost of trams and depots.
"The cost of the surface system includes an extra Docklands loop line, costing about £60 million," it says. "The £850 million cost for the underground option includes a link into Docklands, though this does not serve the Docklands area as fully as the loop scheme."
Referring to the operating costs of both options, it suggests that the on-street system would cost between £1012 million a year, whereas the underground option is likely to cost 10 to 15 per cent more, though this difference would decrease as passenger numbers grew.
In its financial appraisal, Atkins says the surface option would cost less to build and operate, as well as carrying more passengers. This makes it "far superior to the underground option in simple financial terms." But revenues would "not be nearly enough" to cover the capital cost.
Both options would show an operating surplus, with revenue from fares more than covering the cost of operating the system. The surface option would have an operating surplus of around £14 million a year in 2006 compared to £8 million for the underground option.
The economic benefits to public transport users - bus, rail and LRT - are broadly the same for both options at £1,400 million as a result of travel time savings. For others, mainly car users, the economic benefits are estimated at almost £300 million for the underground option.
Over the same 30-year period, the study estimates, the surface option would provide "disbenefits" of just under £100 million, due mainly to the reduction in road space to accommodate the on-street light rail lines.
