The mark II carriages, which were withdrawn from daily use on the Sligo line in 2005, were also used last week on a rail tour run by the railway heritage group, the Irish Traction Society.

Sources said they returned to regular daily use three weeks ago after Docklands station in Dublin was opened to free up enough modern commuter railcars to service the new station. The carriages are due to be replaced later this year by new Intercity railcars.

Iarnrod Eireann spokesman Barry Kenny said the move was designed "purely to bridge a gap of a couple of months between the opening of Docklands and the introduction of the new trains rather than leave a new station lying idle". He denied, however, that this meant there was a shortage of railcars.

Kenny said that although the mark II train was "an old train, there's nothing wrong with it". However, in comments to a Sligo newspaper last year, he said the railcars had "higher reliability and improved lighting and heating" above the older trains.

Meanwhile, Iarnrod Eireann has also admitted that its 59m Dart refurbishment project will be completed two years later than planned after a series of delays. The project now won't be completed until the end of 2008.

The project missed its original December 2006 deadline because the company's contractor, Siemens, was unable to fulfil the conditions of its original tender, which left 16 Dart carriages stranded in Germany for 18 months.

Kenny confirmed that the project had now been further delayed as Iarnrod Eireann had decided to send fewer carriages at a time to Germany for refurbishment.

"We have changed the rate because the majority of our peak-time trains now have eight carriages, " he said.

A spokesman for rail users group Platform 11 said it appeared that Iarnrod Eireann was having problems keeping up with the government's investment plans.

"If they weren't short of trains, why did they have to take some off the Sligo line?"

he said.

He said the group was aware of the carriage shortages, which he said would probably get worse when the Western Rail Corridor is opened. At the moment, Iarnrod Eireann plans to take the railcars for services on the line from its existing fleet.

"We will find out then whether they have ordered enough to keep up with national demand, " he said.