I welcome the Government decision to extend the proposed Light Rail system by linking the O'Connell Street Station with the DART at Connolly Station, and by rerouting the Ballymun line via Broadstone as well as extending it to the airport. This produces a sensible network linked to the DART. It consists of a high traffic density line running eventually from Cabinteely to the airport and Swords, together with a lower traffic density line from Tallaght to Connolly, to be extended in due course to the Docklands as that area is developed.

I do not share the pessimism of those conspiracy theorists who suspect that it is all a ruse to delay the implementation of Luas into the distant future. That pessimism is based on past experience when we were a relatively poor country, short of resources for development. These days Ireland is enjoying a period of unprecedented and sustained growth. The pessimists fail to take into account the scale of future resources now becoming available for investment because of our economic growth - not to mention the proceeds of privatisation, such as Telecom. There are also exciting opportunities for an injection of private capital into this scheme of the kind that has financed the Eastlink and Westlink.

I believe that Luas will now proceed - with only such delays as are necessitated by the additional design work involved.

What I am not happy about is the failure to face the capacity problem on the Cabinteely-Airport route, and the absence of any plan on how to get the Luas through Finglas to Ballymun and the airport. These are serious defects in the project as announced. I also regret the continued insistence on a roundabout routing for the Tallaght line. Running it via Inchicore and the Naas Road, through 2.5 kilometres almost totally devoid of dwellings, will add almost one-third to the journey time for the people of Tallaght, while depriving the population of Kimmage and part of Crumlin of access to rapid transit. Finally, I fear the consequences for north-south traffic through the city of the proposed overground routing of the Connolly-Heuston section of the line to Tallaght. In addition to banning car traffic from O'Connell Street, all other north-south streets across the city will be blocked by trams crossing them at average intervals of as little as 75 to 90 seconds.

The basic problem with the Government's "short tunnel" solution - running Luas underground from St Stephen's Green to Broadstone, with underground stations at College Green, O'Connell Street, and probably Jervis Street/Mary Street - is that at both ends of this short tunnel the trams will be operating on-street. And as the Atkins report has confirmed, on-street operation precludes the use of coupled vehicles. Without coupled vehicles and with a maximum frequency of one vehicle every 2 1/2 minutes, this limits the peak capacity of such a system to 6,500 passengers per peak hour.

I pointed out in these columns over 18 months ago that the extension of the Dundrum line to Cabinteely with "Park and Ride" facilities there - an extension that is a crucial element of the DTI scheme to reduce the volume of commuter car traffic - will within a measurable period increase demand on the key sector of this route to this level of 6,500 in the peak hour, which is saturation level with an on-street or short-tunnel Luas system. This traffic estimate was precisely confirmed by the Atkins report, which agreed that as a result within 10 years or so of opening the route, even 40-metre trams could not cope with the demand here.

But all that was on the basis that at O'Connell Street the line was to turn around and run back out to Tallaght. Now, instead, it is to run to the airport, and eventually to Swords, which will certainly increase significantly the traffic on the key sector from the city out towards Cabinteely. This will bring forward by an unknown number of years the time when 40-metre trams will be unable to cope on this part of the route.

We also do not know what volume of additional traffic will be created by the extension of the Ballymun line to the airport and Swords, but presumably it will be considerable. To some degree, this traffic may be counter-cyclical - with airport staff travelling out of town during the morning inbound commuter rush hour - and into the city during the period of the evening outbound commuter peak. However, the peak period for aircraft movements has consistently been between 6.30 p.m. to 7.30 p.m. - which means that heavy passenger movement to the airport will coincide with the evening outbound commuter rush hour to Finglas and Ballymun. As a result, we could find that the 40-metre trams would from the outset be unable to cope with the demand on the airport route.

The Atkins report is quite unambiguous in its negative verdict on the short-tunnel solution now being adopted by the Government, saying bluntly that, while in purely financial terms a short tunnel would obviously be cheaper than a long one, such a scheme would "do nothing to improve the capacity constraints of the surface option". Accordingly, it goes on, this short-tunnel option "is not favoured . . . as it is limited in its ability to deliver the potential benefits of an underground section in terms of increased capacity".

The report offers two possible solutions to this capacity problem.

One would be the introduction of monster 50-metre vehicles - as against the 30-metre trams CIE was promising us a couple of years ago on the basis of the faulty traffic estimates upon which it designed its on-street LRT project.

The Atkins report's second solution to the capacity problem on this part of the route was the running of a relief tramline down the Grand Canal from Charlemont Bridge to the Grand Canal basin, and onwards across the Liffey and through the north Docklands to Connolly Station and O'Connell Street. But this, of course, would be irrelevant to the problem of handling the airport traffic at what is now to be the northern end of this route - so presumably the only solution we are left is the employment of 50-metre trams on this north-south line, the only question being just how soon the demand will require their introduction.

The other great unknown about the Government's solution is how it proposes to route the airport line between Liffey Junction and Ballymun. The map published to illustrate the Government's intentions is schematic - all it shows is that this part of the route is to be overground and is to run east of Finglas, with an eventual extension to that major suburb. I invite any reader possessing a street map of Dublin to look at it and suggest how an overground Luas could best work its way through the intervening area.

Finally, it should perhaps be added that a careful reading of the Atkins report reveals what has hitherto been obscure: that the corporation plans to introduce a system of "traffic cells" throughout the city centre, "permitting access to O'Connell Street, D'Olier Street, College Green, and a proportion of Nassau Street for public vehicles and delivery traffic only". This "traffic cell" solution has been emphatically endorsed by Tuesday's Government statement, which says it has given the go-ahead for this scheme to block off these main streets to cars.

Incidentally, one of the arguments the Atkins report uses against the underground option is in fact its "inability . . . to provide by its very nature the same benefits as the surface option in terms of enforcing public transport priority within the city centre".

So, as cars are in any event to be banned from these key city-centre streets, there will be virtually no general traffic benefits from placing the trams underground on this short stretch. The only advantages to be gained from the expenditure of some £100 million on the short tunnel appear to be the avoidance of disruption arising from the digging up of the centre city streets to install tram tracks - and a slight improvement in centre city bus timings, because buses will not be slowed down by trams operating on these streets.

The logic of the Government's decision is thus questionable. However, the decision has now been made, and there is no point in prolonging this controversy. Let the work proceed as rapidly as possible.

In this connection it is heartening that the Government intends to proceed without delay with the overground sections of the Tallaght and Sandyford lines, which, on the basis of the Atkins report timetable, would bring the former line to O'Connell Street and the latter to St Stephen's Green by 2002 - the underground link-up to follow, probably by 2002-2005.