A train driver who narrowly averted a catastrophic rail disaster involving hundreds of Belfast-Dublin passengers has been hailed a hero after spotting the tracks were crumbling into the sea.

Keith Farrelly (33) saved countless lives with his quick reaction to signs of subsidence as he was crossing over the viaduct on the main Belfast-Dublin line on Friday evening.

It was just minutes after the Connolly Station-based driver halted his train near Malahide and alerted the Irish rail authorities that around 20 metres of the tracks completely collapsed into the water. A train from Belfast had crossed the viaduct a short time before and a north-bound Enterprise was due shortly after the crash but stopped.

As Mr Farrelly spoke of how his legs “turned to jelly” with the shock of seeing the track collapse, thousands of commuters were today facing the first day of travel chaos. It could be up to three months before the line is fixed and rail passengers from Northern Ireland will face part of their journey, from Drogheda to Connolly, by bus for the foreseeable future.

SDLP MLA Tommy Gallagher, a member of the Department for Regional Development committee at Stormont, said the train driver deserved praise for his quick thinking.

“I would certainly add my own appreciation on behalf of the many people from the north who use that route and will be grateful to the driver for his speedy action,” he said.

“It certainly looks like he had to avert a very serious situation on a train line that is a valued link between north and south.

“The driver has also set a very good example and raised standards for those who are responsible for transporting the public.”

Legs had “turned to jelly”

A train driver who averted a potential disaster involving hundreds of passengers by alerting authorities when he saw a viaduct giving way has told how his legs had “turned to jelly” with the shock.

Around 20,000 passengers on the Belfast to Dublin and northern commuter rail line will have their rail journeys disrupted for at least three months after the “extremely serious incident” when part of a viaduct fell into the sea in the Malahide estuary outside Dublin minutes ahead of trains packed with hundreds of commuters.

What could have been Ireland's worst rail disaster was averted only by the quick thinking of train driver Keith Farrelly (33), who noticed signs of subsidence on the track as he was crossing over the viaduct at 6.25pm on Friday.

He immediately stopped his Enterprise train at Malahide and alerted Irish Rail which suspended services on the northern line.

The heroic driver, from Dublin and based in Connolly Station, described what happened.

“The first inkling I had that something was wrong was when I noticed water splashing up to a high level.

In that location, it's not a normal thing to happen, so I looked at the northbound line and saw that the viaduct was giving way and that the track was hanging,” he said.

“The Dundalk train had just gone over the bridge — it was a very close call. I saw the bridge start to collapse as I was going over it, it was a scary situation.

“It was pretty hairy I tell you . . . it was surreal. I was just relieved that we got past.

“I couldn't believe what I was seeing, the waves were coming up over the side of it. I thought I was seeing things.

“Immediately then I could feel the ballast moving on the line beneath my train and realised the danger we were in. My defensive driving training then kicked in and I decided to coast the train in, lightly braking, so I didn't put pressure on the track, and ensured we had momentum to get us safely to the station.

“When I arrived at Malahide Station I protected both lines, and alerted Control. It was such an unreal sight, I started thinking to myself ‘did I really see that', but when I walked back I saw it clearly, and my legs just went to jelly with the shock.

“I'm just glad that all of us on board walked away from it safely.”

Engineers had inspected the viaduct on the main Dublin to Belfast line only last Tuesday, but heavy rain in the following days may have played a part in a 20-metre section of the viaduct falling into the fast flowing waters of the estuary just before 6.30pm on Friday.