Passenger safety was not put at risk by door faults on the Enterprise train, Translink said.
A safety licence was suspended in the Republic of Ireland after the doors opened unexpectedly while the train was moving. A notice was served by the Railway Safety Commission in Dublin.
Two separate incidents have occurred in the last two months on the service operated jointly by Translink and Irish Rail.
Translink chief executive Chris Conway told Stormont's regional development committee: "Passenger safety was never put at risk."
In one instance this month, no passengers were near the door when it partially opened as staff made arrangements to load a catering trolley in Newry.
In another case last month, a train official heard air coming through the door and it was later discovered that a bolt was faulty.
Translink carried out a detailed technical investigation and review of the door mechanisms.
It has introduced a significant Enterprise train refurbishment programme which it said will ultimately provide a much enhanced quality of service to passengers travelling on this important cross-border route.
The redesign includes new livery, replacement of the passenger information system, a new electronic reservation system, security cameras and redesigned interiors.
In 2013/14, almost 840,000 people travelled to Dublin on the Enterprise.
Union Unite has blamed public transport company Translink for "mismanagement" in overhauling the Belfast to Dublin flagship service after claiming the refurbishment was delayed by six weeks.