The service operates between Belfast and Dublin
Northern Ireland Railways has revealed that its premier service, the Belfast to Dublin Enterprise, is losing money and needs a £500m overhaul.
It has also emerged that on average an Enterprise train breaks down every eight days.
A standard adult return fare between Belfast and Dublin currently costs £36.00.
Regional Development Minister Conor Murphy has admitted there are problems with the service.
"There is always room for improvement - you can never be complacent and say a service can't be improved," said Mr Murphy.
"And some of the flaws relate to the structure the service operates on, which is the responsibility of ourselves and the two railway companies, to try and decrease the time between the two cities.
"Travel on the train between the two cities is hugely important for both administrations."