Dublin's two Luas light rail lines from the city centre to Sandyford and Tallaght are being built to different standards, the Railway Procurement Agency (RPA) has acknowledged.

Luas was originally to be built to a uniform specification with the Sandyford line being upgraded to a metro standard at a later, unspecified date.

In order to avoid first opening the Sandyford route as a tram line and then closing it while it was upgraded, the RPA decided to build the Sandyford line to a metro specification in the first place.

A spokesman for the RPA insisted last night that the gauge of the track on both lines would be the same, to permit interchangeability of trams in the future.

He acknowledged, however, that the 40-metre long trams on the Sandyford line would not be able to travel on the Tallaght line, even if the the two routes were eventually connected.

While the shorter and lighter 30-metre Tallaght trams would be able to travel on the Sandyford line, the two routes are likely to remain separated because the trams are not fully interchangeable.

The difference in specification between a tram line and a metro line is in the length of the carriages, the width required for turning movements and the strength of bridges and foundations.

The RPA acknowledged the situation yesterday in response to criticism from the former Taoiseach, Dr Garret FitzGerald, who contends that Luas has "turned into an unmitigated disaster".

Writing in this newspaper today, he says this was largely because of a lack of economic expertise, but also because of traffic upheaval and the difficulty in transferring passengers from Luas to metro.

But the RPA rejected a key criticism that the Ranelagh to St Stephen's Green section of the Sandyford line would be abandoned in favour of an underground metro link, starting south of Ranelagh.

Meanwhile, the Minister for Transport, Mr Brennan, said yesterday he has full confidence in the selected Luas operator, Connex, after the group lost a franchise to run some trains in the UK.

Britain's Strategic Rail Authority sacked Connex for its poor financial management and said a state-run service would take over for the first time since the railways there were privatised.