Irish Rail says the partial derailment of a train in Co Wicklow could have resulted in fatalities.
The incident, involving a four carriage DART train last night, is being blamed on vandalism.
Thirty-three passengers escaped injury after the train came off the tracks after striking rocks near Bray at 11.15pm last night. Nobody was injured.
It happened around 100 metres north of the exit from the first tunnel on the rail track from Greystones to Bray.
Gardaí at Bray said a number of rocks were found on the railway track causing the train to derail.
They said: "From initial investigations carried out by An Garda Síochána and Irish Rail it appears that rocks may have been dislodged by unknown persons."
They are appealing for anyone who may have been walking on the Greystones/ Bray cliff walk between 8pm and 12 midnight last night to contact them.
They are also asking for anyone who may have seen a group or individuals at the start of the cliff walk in Bray or the entrance to the cliff walk in Greystones to get in touch them.
Anyone with information is asked to contact Bray Garda Station on 01-666 5300, the Garda Confidential Line 1-800-666-111 or any Garda Station.
Stretches of the cliff side path between Bray and Greystones are guarded on one side by walls made up of large boulders and blocks cemented into place.
There are also sections of wire and steel mesh attached to the slope and rock faces at certain points to act as a stabiliser for sheer cliffs and above the railway lines on some points to act as nets to catch rockfalls.
The area where the boulder came from showed no sign of rockfall or landslide.
Irish Rail said its investigations showed the dislodged piece was a coping stone from the top of the boundary wall on the cliff side.
There is no CCTV along that section of the cliff walk.
Irish Rail chairman Phil Gaffney described the suspected vandalism as reprehensible.
“Actions like this are extremely serious and reprehensible and could have resulted in serious injury to the driver and passengers,” the chairman said.