The decision to build the Sandyford and Tallaght Luas lines to different specifications may have implications for the proposed Dublin airport metro, it emerged yesterday.
According to the Railway Procurement Agency (RPA) the Tallaght and Sandyford lines are built to the same gauge, and trams on both routes are fully interchangeable.
However, the agency has acknowledged that the inbound and outbound tracks on the Tallaght line were built closer together than the inbound and outbound tracks on the Sandyford line.
The Sandyford route was built with a wider median width to allow it to accommodate wider carriages which have more capacity than narrower tram cars.
The result is that should the RPA decide to opt for wide carriages for the proposed Dublin Airport metro, those carriages would be able to run on the Sandyford line.
But the narrower median on the Tallaght line means that wide carriages would "probably be unable to use that route", according to the RPA.
This has implications for the Dublin metro as the RPA must decide to opt for high-capacity wide carriages on the new metro, or lower-capacity narrow carriages which can also operate on the current Tallaght route.
As most platforms are level for only 40 metres, the ability to substantially increase the length of the Tallaght trams, and so capacity, is limited.
According to the RPA spokesman, Mr Ger Hannon, other factors including speed, frequency and width as well as length of train were all variables which could be adjusted to increase capacity. But he accepted that other considerations on the Tallaght line, such as the amount of on-street travel and level crossings, restricted these options.
According to Mr Hannon, current thinking was that the airport metro would not link up with the existing Tallaght Luas line. The agency was exploring the idea of a "travelator" linking Tara Street station with an underground in the vicinity of D'Olier Street.
