Hundreds of lives were put at risk after faults on a major cross-border commuter line were ignored — three years before it collapsed into the sea, a leaked report has revealed.

A 20m section of the Malahide viaduct on the main Belfast to Dublin rail line collapsed last August — minutes after a passenger train packed with commuters had crossed over.

Tragedy was only averted because the driver used his emergency training to coast the train over the embankment.

Now leaked documents reveal how the potential catastrophe, which caused travel chaos to thousands of Ulster commuters, could have been avoided.

A survey of the seabed in 2006 found evidence of serious erosion around pillars.

But Iarnrod Eireann (Irish Rail) failed to heed the warning because its engineers did not understand how the viaduct structure worked, its report into the incident reveals.

The bridge crumbled into Broadmeadow estuary near Malahide on August 21 last year.

A train from Belfast carrying dozens of Northern Ireland passengers had crossed the viaduct a short time earlier.

Northern commuter services were among the worst affected.

Passengers using the Belfast to Dublin Enterprise service could only travel as far as Drogheda before being transferred to buses for the remainder of their journey.

Translink later said it lost up to 60% of cross-border rail passengers and about £2m revenue.

The report found that the seabed was being washed away from at least 2006, undermining the foundations of a pillar that supported the track.

Engineers failed to realise the viaduct's foundations had to be maintained because the knowledge had been lost as staff retired or moved to other positions.

The report said: “The collapse of the structure was due to the undermining of one pier's foundation caused by ‘scour' erosion.”

It contradicts evidence given by company officials to the Dail transport committee last September. Chief executive Richard Fearn then told TDs and senators: “That survey (2006) showed no evidence of scour (erosion) and gave a clean bill of health for the following six years.”

But the 2006 survey said, like all bridges in water, the structure was prone to erosion.

And it found that the seabed was being eroded near the support pillars.

Iarnrod Eireann last night admitted safety staff had not known the foundations of the viaduct needed to be maintained.

A spokesman said: “Our focus now is on learning from the key lessons of this most serious accident.”

A Department of Regional Development spokesman said it could not comment until the report had been officially published.