KiwiRail says high temperatures may have caused overhead wires in Wellington to sag, causing a rail stoppage for most of the region on Tuesday.
Photo: RNZ / Paloma Migone
Early inquiries show Tuesday's rail stoppage in Wellington was caused when a moving train became tangled in the overhead wires that provide it with electricity.
KiwiRail said this might have been caused by the wires sagging too low due to expansion from the heat. But it might equally have been caused by damaged equipment on the train itself.
The outage lasted four and a half hours and halted all electric trains, leaving thousands of commuters stranded.
A $98 million programme is under way to ensure that overhead wires that provide Wellington network trains with electricity remain taut.
KiwiRail head of asset management David Gordon said the old system involved keeping wires on fixed poles with fixed tension, so there was little to stop them from sagging if they expanded in the heat.
He said the three to four year project, which began last year, aimed to replace this with a system using pulleys and weights to make sure wires stayed at the same height regardless of temperature changes.
KiwiRail runs the track and wires and has six staff investigating the accident.
Mr Gordon said the part of the train that snagged was a pantograph - a zig-zag-shaped piece of equipment that rises from the top of the train to the wires.
He said there might be two reasons for the pantograph to become snagged with the wires.
"The first explanation is that the pantograph somehow failed and when it reached the junction, the damaged pantograph pulled down the wires," he said.
"The alternative, equally credible explanation, is that the wires sagged due to the heat and the pantograph caught on a sagging wire and pulled it down."
Mr Gordon said investigators were still trying to find the answer to this issue.
In the meantime, extra poles would be put in place to prop up the overhead system, just in case sagging wires turned out to be the reason for the crisis.
Besides the KiwiRail inquiry, the train operating company, Transdev, would also investigate.