The Transport Accident Investigation Commission (TAIC) has confirmed it is investigating reports of two derailments which involved the same KiwiRail freight train on the Main South Line last night.
The incident has resulted in closure of the line, affecting up to 12 freight services.
It was reopened at about 6.30pm.
A commission spokesperson said the derailments reportedly involved the train’s rolling stock, which remained upright "but sustained serious damage".
There were no injuries to people.
It had collected evidence which showed the train appeared to derail around the time it arrived in the Dunedin rail yard last night, about 10pm.
It had also received reports that the same train had derailed earlier in the night, near Allanton, but this was yet to be confirmed, the spokesperson said.
An investigator was currently on site looking for signs of the first derailment, but any evidence of this was yet to be found.
Derailments were always a safety concern, the spokesperson said.
"Double derailments involving the same train are unusual.
"After the [reported] first derailment, this train passed over numerous level crossings and through tunnels."
The commission opened an inquiry when it believed the circumstances of an accident or incident had or were likely to have "significant implications" for transport safety.
The initial focus was on gathering evidence that could disappear or change.
"TAIC’s investigators will also be seeking and securing electronic and documentary records and interviewing the people involved.
"We are interested in the locomotive and its rolling stock, their individual and type histories, performance, maintenance, equipment and design."
A team of three had been appointed to investigate and rail inquiries mostly took between 18 months and two years, the spokesperson said.
A KiwiRail spokesperson said they were investigating possible causes after two derailed wagons were found at the Dunedin rail yard yesterday.
"As it was not initially clear where the derailment had occurred, the Main South Line from Dunedin south was shut as a precaution until it could be inspected and NZTA and TAIC were notified.
"It is now not clear if there was a prior derailment."
Safety was always the priority and KiwiRail took all derailment incidents seriously, the spokesperson said.
Up to 12 freight services were affected by the temporary closure of the line.
An NZTA Waka Kotahi spokesperson said they had been informed of an incident and were aware KiwiRail and the TAIC were looking into it.
Dunedin Railways general manager Rebekah Jenkins said a Taieri Gorge train service was scheduled for tomorrow.
They were "hopeful" it would still be able to operate, subject to track availability, she said.
