Supporters of underground rail schemes in Dublin, such as MetroLink, bemoan the absence of a subway when compared to what they regard as similar European capitals.

But cities with established underground systems have far higher trip volumes along routes, reflecting higher densities of population and employment, than has been permitted to emerge in the Dublin area.

Cities with high density and bigger populations often have airports which are much further from city centres, not an issue in Dublin

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Underground rail networks in large European cities like London and Paris were built in the later decades of the 19th century and pre-date the spread of automobile ownership and use. These cities have far higher population density than modern Dublin.

Cities with high density and bigger populations often have airports which are much further from city centres, not an issue in Dublin, which airport, at just 10km, is one of the closest to the centre of any major city in Europe. This makes provincial bus routes feasible, serving both airport and city centre.

The only underground public transport facility in Dublin is the Port Tunnel, parallel to the intended route of MetroLink and busy with buses from the central area direct to the terminals in 25 minutes.

The airport attracts patrons from all 32 counties and is the busiest bus station in Ireland.

The proposed MetroLink, for which not even a firm cost estimate is available, could cost €16bn or €17bn, even more according to sceptics.

Since there is no cost estimate, there can be no cost/benefit analysis, and the Government is relying on studies undertaken not by the Departments of Public Expenditure or Finance, but by consultants working for the project promoters, in this case agencies of the Department of Transport.

Contrary to popular perception, the share of the Republic’s population living in Dublin city and county has not increased substantially in recent decades

These studies, as well as lacking definitive cost figures, must contend with a suspicion that the promoters are marking their own homework and that a fully independent economic evaluation might come up with an unfavourable answer.

There is comprehensive evidence, well-documented around the world, that forecasts of cost are typically too low and of patronage too high, when they are prepared by, or for, project promoters.

Contrary to popular perception, the share of the Republic’s population living in Dublin city and county has not increased substantially in recent decades. The share has been stuck just under 30% which may sound a lot, but is not unusual for the biggest city in a small country. In Singapore, the corresponding number is 100%.

Helsinki metropolitan area

In Finland, for example, where total population is around the Irish figure, the Helsinki metropolitan area has about the same percentage of the national population as the Dublin metropolitan area.

Since the census of 1979, the Dublin city and county population has grown at a pace a little below rates of growth outside Dublin.

The fastest-growing populations in Ireland in recent decades have instead been in the rest of the greater Dublin area (GDA), counties Meath, Kildare and Wicklow.

Of the 847,000 increase for the GDA, 353,000 has been in the three outer counties, close to half of the entire region’s total extra residents.

Every single Irish county has seen some population growth in the most recent census, but there is no basis for seeing the slower growth counties as suffering at the hands of excessive expansion in Dublin.

Low-density sprawl creates a hostile environment for public transport, especially for underground railways

The excessive expansion has been in Kildare and Meath in particular. Even within the county of Dublin, the most rapid growth has been in more distant suburbs of Fingal and the southwest of the city.

Drogheda, 50km away, has been the fastest-growing town in Ireland. This is urban sprawl.

Low-density sprawl creates a hostile environment for public transport, especially for underground railways which are the most expensive technology.

The least costly in a low-density sprawl is the encouragement of bus transport through measures, such as priority in the kerbside lanes and ultimately congestion charging designed to get cars out of their way at peak times.

Port Tunnel

In Dublin, the only example of congestion charging for mobile car users is in the Port Tunnel, where inbound traffic faces a stiff toll in the morning peak, reversed and imposed on outbound users in the evening.

The tunnel is free for buses and for large trucks, encouraging them to avoid the centre and head for the M50 and their ultimate destinations around the country. All other road tolls in Ireland are flat charges undifferentiated by time of day.

If Dublin’s density had risen sharply over the last 50 years, prospects for public transport would be better. Buses, plus congestion charging, is now a better option than MetroLink.