An investigation is to take place on the collapse of part of the main Dublin to Belfast line at Malahide in Co Dublin.
A spokesperson for Iarnród Éireann said that the section of the bridge that collapsed underwent a structural examination this week.
The inspection was carried out on the line by an engineer on Tuesday and no issues were identified.
Iarnród Éireann says it will be at least three months before the damage is repaired.
It says the incident had the potential to be a serious tragedy and will be trying to establish how the collapse happened.
The 18.07 Balbriggan to Connolly Station train was crossing the viaduct on the Broadmeadow Estuary between Malahide and Donabate at around 6.20pm yesterday evening.
The driver noticed subsidence on the rail line opposite, which he reported and all services were immediately stopped.
A 20-metre section of the viaduct then collapsed into the water.
There were between 40 and 50 people on the train, which had just crossed over but no-one was injured.
Iarnród Éireann says lines are inspected three times a week by a patrol man and they are given a structural assessment by an engineer every two years.
Rail Users Ireland has said it is extremely concerned following the collapse.
The group says the incident raises 'serious questions of Irish Rail's maintenance and inspection regime'.
It also called for cash refunds for all weekly, monthly and annual ticket holders that are unable to travel.
Disruption to services
Iarnród Éireann has said trains will only operate between Drogheda and Skerries again tomorrow.
The company has advised customers travelling between Dublin city and all stations north of Malahide to use Dublin Bus or Bus Eireann services.
Belfast Enterprise services will operate with train services between Belfast and Drogheda, and with bus transfers between Drogheda and Connolly.
DART services between Malahide and Howth Junction are operating normally.
No arrangements have been finalised for services on Monday but Iarnród Éireann said it was working with Dublin Bus and Bus Éireann to provide enhanced services on existing routes serving the communities affected by the disruption.
For further information go to the Iarnród Éireann website