Dart services between Dalkey and Bray were suspended on Tuesday morning leaving commuters facing long delays.
Irish Rail said the suspension was “due to an incident of vandalism which has damaged overhead lines”.
A pantograph which connects the train to the wires was also damaged, the company said.
Irish Rail said southbound Darts would turn around at Dalkey and go back to the city. “Knock on delays can be expected due to congestion,” the company added.
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Passengers on route between Rosslare, Co Wexford to Dublin Connolly are being offered a bus connection from Bray onwards.
Irish Rail said it was working to rectify the situation as soon as possible.
Meanwhile, a crash on the M50 northbound created significant delays for morning commuters on the road.
A car overturned on the right-hand lane of the motorway between Junction 4 Ballymun and Junction 3 Santry.
Dublin Fire Brigade posted a photograph of the overturned car on its social media, shortly after 8am. Lanes Two and three on the northbound carriageway were closed.
Traffic congestion in the area was significant with delays of 15 minutes coming from Junction 5 Finglas.
Traffic was advised to slow down on approach and to expect delays.
