The underground option could carry a substantially higher number of passengers than the surface option, with a higher level of comfort and reliability, the Atkins study says.
More seats would be available and it would also be less constrained in catering for future demand.
The study notes that travel demand in Dublin has been increasing rapidly in recent years because of economic growth. This trend is forecast to continue, giving rise to concerns that the surface LRT system currently proposed would not be able to meet future demand.
But the study concludes that the surface option serving Tallaght and Dundrum has "sufficient capacity to carry forecast passenger demand for the foreseeable future, at least until the year 2025." These forecasts take account of recent economic growth and likely future growth.
For the purposes of the study, the consultants calculated the maximum capacity of the surface option at 6,500 passengers per hour in each direction.
This was based on 40-metre vehicles and a service frequency of 2 1/2 minutes in each direction.
However, it warns that as the surface system is extended, there is a potential for overloading in the peak hour in the future unless the implementation of the extensions is carefully planned and measures are taken to increase the capacity of the system.
Subject to these caveats, it says a full surface LRT system serving Ballymun, Cabinteely and Tallaght as well as Clondalkin, Docklands, Finglas and Swords, "is capable of accommodating passenger demand up to between the year 2025 and 2030, assuming current growth trends."
The following are some of the key findings:
In the peak hour in 2006, the surface option is expected to be carrying 13,000 passengers compared to 12,000 for an underground.
During the off-peak period, the surface option would attract nearly 50 per cent more passengers than the underground option.
The estimated annual passenger demand in 2006 is 23 million for the underground and 29 million for the surface option.
Both options would have broadly the same effect of increasing the number of passengers travelling by rail by 4 per cent and reducing the numbers travelling by car by 1 per cent.
Almost three-quarters of passengers on the LRT lines - surface or underground - would have previously travelled by bus.
The surface option would serve short-distance trips better, while the underground option would be more attractive for commuting from the suburbs.
"It is important to stress that light rail will only account for a small proportion of total trips in the Greater Dublin area. In 2006, the Tallaght and Dundrum lines will account for about 5 per cent of total trips made in the morning peak hour and for about 15 per cent of total public transport trips across the full network.
"Effective action to tackle Dublin's traffic problems cannot rely on LRT by itself; it requires action on a broad range of measures recommended by the Dublin Transportation Initiative," the study says.
These include a network of quality bus corridors (QBCs), upgraded DART services, the provision of "park and ride" sites and completion of the M50 bypass.
In terms of "extendability", the Atkins study notes that the ability of the underground option to offer higher passenger capacity and operational efficiencies would increase as the network is extended. But it says the surface option can be extended more flexibly and at a lower capital cost.
The main disruption would occur during construction of the Tallaght and Dundrum lines. But provided that other elements of the DTI strategy are in place, the network "can be extended with relatively little additional impact on the operation of the rest of the transport system."
The study says the main disadvantage of the surface option is the potential constraint on its passenger-carrying capacity on certain individual route sections at the busiest times of the day. But it suggests that this could be overcome by having longer (40-metre) trams.
