Iarnród Éireann says no DART services are operating between Landsdowne Road and Bray tonight because of a 'low speed derailment' at Dún Laoghaire.
Services are only operating from Lansdowne Road to Howth/Malahide.
The company hopes to have a full service in operation tomorrow morning.
Part of the front carriage of the 4.45pm Howth to Bray service derailed just outside Dún Laoghaire after around 6.15pm.
There were about 400 passengers on board and there were no reports of any injuries.
Services suspended between Lansdowne & Dun Laoghaire UFN due low speed derailment at Dun Laoghaire. Update to follow.— Iarnród Éireann (@IrishRail) September 13, 2017
A spokesperson for Iarnród Éireann said an evacuation plan was put in place with passengers walked back to the nearest station by Iarnród Éireann staff.
Dublin Bus is honouring passengers' tickets, and Rosslare services will be bus transfers.
Iarnród Éireann said it, the Rail Accident Investigation Unit and the Commission for Rail Regulation will all be investigating the incident.