The refurbished Enterprise train will be back in service from today, Translink said.
A safety licence was suspended in the Republic of Ireland after passenger doors opened unexpectedly while the train was moving. A notice was served by the Railway Safety Commission in Dublin.
Three separate incidents have occurred in the last two months on the service operated jointly by Translink and Irish Rail.
Translink carried out a detailed technical investigation and review of the door mechanisms.
Ian Campbell, a general manager, said: "Safety is a top priority for everyone at Translink and we are committed to maintaining our high standards in this area.
"When these incidents occurred, all the appropriate safety and operational procedures were carried out. We immediately addressed the issue, removed the train from service and reported the event to the relevant safety authorities.
"We would strongly reassure our passengers and the wider public that there was no imminent danger for our customers travelling on board as a result of these two unrelated door faults.
"We have worked closely with the Railway Safety Commission and having satisfied their concerns, the upgraded train will now be returned into service this afternoon.
"We will continue to collaborate with the Railway Safety Commission as we work to bring this significant Enterprise train refurbishment programme to fruition which will ultimately provide a much enhanced quality of service to passengers travelling on this important cross border route."
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.