A viaduct on one of the country's busiest railway lines collapsed because workmen carrying out safety checks did not know how to properly assess it, an accident report has revealed.

For more than 40 years repair work focused on pillars holding up the track over the Broadmeadow estuary, north Dublin, rather than on the causeway they were built on. Irish Rail also said an engineer who checked the viaduct near Malahide days before its near catastrophic failure only looked at the piers and not underwater foundations being eroded.

Company spokesman Barry Kenny said significant grouting work to protect the superstructure took place in 1967 up to 2m below sea level. But investigators warned that since then new engineers joining the company did not know there were two separate parts to the viaduct - piers resting on a causeway were not embedded in the bedrock.

Fergus O'Dowd, Fine Gael transport spokesman, accused Irish Rail of jeopardising hundreds of passengers. He asked: "How are we supposed to have faith in the rest of the rail network, when Iarnrod Eireann's excuse for this fiasco is that key staff members had retired?"

Tommy Broughan, Labour transport spokesman, said the investigation had uncovered astonishing gaps in maintenance work. He said: "The near-disastrous event raised serious question marks about the safety procedures and culture at Iarnrod Eireann and, in particular, at the Railway Safety Commission."

Both opposition TDs called for Irish Rail chiefs to be called before the Oireachtas Transport Committee to explain the incomplete maintenance checks.

Rail chiefs had been warned about the state of the viaduct by the Malahide Sea Scouts days before the accident on Friday August 21 last year after a canoeist saw a stone washed away.

"However a misunderstanding appears to have developed so that the engineer delegated to inspect the viaduct on 18th August was looking primarily for cracks or missing stones in the pier structure rather than in its foundations," the investigation found.

The driver of a commuter train spotted the collapsed bridge at about 6pm and raised the alarm.

Irish Rail said the engineer sent to check the viaduct days before it fell into the sea found dressed stonework needed repointing and some cracked stones on a number of piers. None of the faults spotted were considered serious and the engineer thought they explained the erosion warning from sea scouts. The line remained closed for three months after the accident. Irish Rail also denied it had been warned about serious erosion on the viaduct in 2006.