The newest Enterprise train on the Belfast-Dublin service has been banned from the railway tracks because of a fault that lets doors open mid-journey.
Safety chiefs in Dublin served a prohibition notice on NI Railways, banning the Enterprise train from operating in the Republic of Ireland.
The new train - which reaches a maximum speed of 90mph - was unveiled as part of a £12.2 million refurbishment programme and started operating in November.
But doors opened unexpectedly during two journeys on December 17 and January 6, once at Newry and the other at Dublin.
A Translink spokesperson said an "extensive safety approval process" had taken place before the Enterprise train was authorised to operate on NI Railways and Irish Rail networks.
"On December 17th the Enterprise was taken out of service following a report of a door fault. Following a full check of the Enterprise fleet, the train was returned to service on December 19th," she said.
"On January 6th a further door incident occurred on the Enterprise which was immediately addressed by a member of staff on board and the train was removed from service.
"Translink treats safety as paramount and internal investigations have highlighted that the two door incidents are unrelated in nature and at no stage were passengers in any immediate danger."
A technical investigation and remedial action is currently underway across the Enterprise fleet involving specialist door contractors and the train door manufacturer.
In the meantime, the Enterprise train will remain out of service - check here for the latest travel updates.
The Republic's Railway Safety Commission spokeswoman told the Irish News: "The wrong side door failures posed an immediate and substantial risk to the safety of persons."
"The prohibition notice is independent of the RSC's approval on the newly re-furbished Enterprise trains. The doors or door circuit were not part of the re-furbishment project."
She added: "NI Railways is addressing our safety concerns and the RSC awaits the submission of evidence to demonstrate that the risk to the safety of persons is as low as reasonably practicable."
Yesterday, railway workers said the overall refurbishment programme has been delayed by six weeks.
Translink is the trading name for the Northern Ireland Transport Holding Company, which is the parent company for NI Railways, Ulsterbus and Metrobus. The annual report for the year to March 30, 2014 gives a broad ranging summary of the financial performance of the group based on the published accounts.