Train suspended on washed-out track for 12 minutes
The first carriage of a train with 150 passengers on board was suspended over a large hole for 12 minutes, the Rail Accident Investigation Board has said.
The board said the driver probably prevented a derailment when he took the initiative and reversed the train.
The hole was created when 150 tonnes of embankment near Lisburn, County Antrim, were washed away by rain in June 2012.
While the engine and a carriage crossed the drop, most passengers were in the other carriages, which stopped short.
Rail investigators only found out when they were tipped-off seven weeks later that 10 metres of track were left unsupported when part of an embankment had been washed away.
NI Railways only began a formal investigation after the Rail Accident Investigation Board (RAIB) asked them about what had happened.
The RAIB, which probes all incidents on the railways, said the washout of the embankment could have led to a derailment.
NI Railways did notify the RAIB about the embankment failure, but did not mention that a train and passengers had been involved.
It was seven weeks before the full facts emerged about the incident, which happened at 07:00 on 28 June last year.
The train was travelling at 24mph when it ran over the 10-metre section of track that had been washed away by torrential rain.
It was a `golf special` on its way to the Irish Open at Portrush travelling along the Antrim branch line, which is now used predominantly for engineering traffic.
The RAIB report also said NI Railways was not aware of the flood risk in the area and had no plan for dealing with heavy rainfall.
The torrential downpours the night before the incident had flooded 1,600 homes in Belfast and the Coca Cola bottling plant in Lisburn close to the scene.
The report said that NI Rail had "no procedure in place to guide on-call staff about what they should do in the event of heavy rainfall" and had no concerns about the Antrim branch line because there had never been a landslip there before.
The Rivers Agency had published a flood risk map for Northern Ireland and updated it in November 2011. It showed the area around the embankment to be at risk from flooding.
The previous year the Rivers Agency had also hosted a workshop on the flood map for owners of infrastructure to help them understand the risk to their assets.
Northern Ireland Railways were invited but did not attend "because the invitation, sent by email, ended up in a spam folder".
The agency also said it sent disks to NI Railways in January 2011 with flood mapping information and asked for feedback. NI Railways said it had no record of receiving them.
'Not properly reported'
The RAIB report found that following the incident at Lisburn, the golf special had been allowed to continue to Portrush without any checks on crew or the train.
As well as an investigation into what had happened, NI Railways also began a separate inquiry into why it had not been properly reported to the RAIB.
The RAIB report said this was due to a combination of several factors.
Amongst them was that no-one had been injured or rolling stock damaged and it was a busy period for NI Railways.
The responsible manager in Translink's Safety department said he was not aware that the front of the train had run over the unsupported track and did not think the incident required a formal investigation.
It had happened just before key staff had gone on holiday and there was `"insufficient senior management oversight" of the events.
Delays meant evidence was not properly collected and the train's black box data recorder was overwritten before the information on it could be downloaded.
As well as informing the RAIB, NI Railways is also meant to tell the Department of Regional Development and the Health and Safety Executive. It did so, but in all three cases it neglected to mention the involvement of passengers.
"The notification to all three statutory bodies were different in the wording but were consistent in omitting to say anything about the involvement of a train in the incident and the significant risk that resulted," the RAIB report said.
"As a result of this omission the full circumstances of the incident were not conveyed to the statutory bodies."
Speaking on BBC Radio Ulster's Talkback programme, Ciaran Rogan from Translink praised staff on the ground and said that the company had not tried to cover up the seriousness of the incident.
He said they had followed industry procedure when reporting what happened.
"When it comes to an incident, there's a set of procedures which we have to follow and the people on the ground took out the manual, went through the categories of incident and they allocated it according to the category which they saw fit," he said.
"RAIB have clarified that the call that was made, the categorisation which was made, is not what they would have done and they have clarified that for the entire industry."