A 610m-long rail bridge over the Rangitata River, South Canterbury, sags after flood water washed away one of 34 piers.
KiwiRail could have prevented the partial collapse of a key South Island rail bridge that could have caused a catastrophic derailment, the Transport Accident Investigation Commission (TAIC) has found.
The report into the collapse of a section of the Rangitata Rail Bridge in 2024 identified three core safety issues:
KiwiRail's inspections were not in line with its own standards
There was no plan to mitigate the risk of the river scouring the bridge's base
KiwiRail's response to weather events failed to take into account specific risks, leaving the bridge open even after the weather met red-alert levels.
Had the bridge not been closed after a member of the public alerted KiwiRail, TAIC said it was "virtually certain" a derailment would have happened with "catastrophic" consequences, including serious injuries and significant damage to the train and the environment.
The 90-year-old bridge, Bridge 57, was constructed in 1936 and is one of about 1600 bridges KiwiRail manages across the rail network.
It crosses the Rangitata, one of Canterbury's largest braided rivers, and is an important rail transport link for much of the South Island, with about 45 freight trains crossing it each week
In April 2024, amid heavy rainfall and flooding, floodwater scoured away the riverbed around one of the massive 120 tonne concrete piers that supports the 610-metre long bridge, causing the pier to sink , then collapse.
KiwiRail only found out when a member of the public raised the alarm.
Chief investigator Louise Cook said it was only down to "sheer timing luck" the outcome was not far worse.
"If a train had traversed that bridge it's virtually certain the middle support of the bridge would have been lost completely, the train would have derailed entering the river and of course the consequences of that are catastrophic.
"It's very likely there'd have been injury to the train crew, the loco[motive] and wagons damaged, possibly environmental damage to the river and of course further structural damage to the bridge itself."
TAIC chief investigator of accidents, Louise Cook.
TAIC found KiwiRail was aware of the risk of scouring at the bridge and of the heavy rain, river flow and flood surge expected to reach the rail bridge on the morning of 12 April.
Multiple conference calls were held with members of KiwiRail's infrastructure and train control groups, and the company was aware the river flow exceeded KiwiRail's own severe weather red-alert over night.
The peak flood flow was expected to hit the bridge at about 8am on the morning of 12 April.
A northbound freight train crossed the bridge just after 6am, and another freight train went across at 8.30am.
A inspection was carried out at 8.53am, which included crossing the bridge in a hi-rail vehicle (which can drive on both rail and road) and taking photos of the river level, but did not inspect the bridge's piers or the debris mounting at some of the piers.
The inspection was completed by 9.41am, and the inspector reported the line was safe to remain open.
The commission estimated the collapse happened around 10.30am.
At 11.28am, a member of the public contacted KiwiRail to alert them to the partial collapse of the bridge.
The next train was scheduled around 2.30pm.
The commission estimated the collapse happened around 10.30am.
TAIC found inspections of the bridge had not met KiwiRail's standards, creating a missed opportunity to identify and respond to the flood risk.
Investigators found KiwiRail conducted two of the three types of inspections required - general and detailed - though noted the information reported was inconsistent.
However, KiwiRail could not provide evidence of any engineering inspections of the bridge, and only provided evidence of one engineering inspection being completed across the entire rail network between July 2016 and February 2024, investigators said.
TAIC also noted KiwiRail's lack of a formal audit process.
It found that if an engineering inspection had been carried out it was likely the risk of the pier failing due to the river's scour would have been better understood, and if an audit system in line with KiwiRail's own quality management system had been put in place, the inconsistencies in its reporting would have been identified and improved.
TAIC also concluded it was very likely the river was above the red-alert level when the two freight trains and the inspector crossed it, presenting a "significant and unmitigated risk" to the train drivers and track inspector.
Other issues identified by TAIC included the lack of a specific plan to deal with the identified scour risk, which can be particularly acute in braided rivers as they shift and move, constantly changing the pressure points, and KiwiRail's failure to have specific responses to the risks from adverse weather events.
The report said KiwiRail w…
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