Iarnród Éireann has said vandals caused a rockfall that led to a train derailing near Bray station in Co Wicklow last night.
Results of early investigations suggest that rocks were dislodged from a boundary wall above the railway line at Bray.
The line reopened at 9.50am after being closed overnight, and all services are now running normally.
Last night's 11.10pm DART service from Greystones in Co Wicklow to Connolly Station in Dublin hit boulders about 1km south of Bray station.
Two sets of wheels on the four-carriage train came off the track as a result.
There were 33 passengers and the driver on board, however nobody was injured.
Up to 1,500 passengers experienced travel disruption this morning as a result.
Iarnród Éireann chair-designate Philip Gaffney said it was a very dangerous act, which could have resulted in fatalities.
The company is asking for anyone with information relating to the incident to contact gardaí as they are looking into the matter.