Preliminary investigations suggest Tuesday's rail stoppage in Wellington happened when a train became tangled in the overhead wires that provide it with electricity.
That might have happened because the wires were sagging too low.
The outage lasted four-and-a-half hours and stopped all electric trains from leaving the capital's central railway station.
Other trains could not come in, and thousands of people were left milling around the station waiting for hastily arranged buses to take them to their destination.
The chaos was described by the train operator Transdev as a worst case scenario - an almost total halt of services right though the evening rush hour.
KiwiRail, which runs the track and wires, has begun investigating the cause of the problem.
It said preliminary results showed the outage was caused by a part of the train snagging with over-head electric wiring close to the station.
The part that snagged was a pantograph - that is the zig-zag shaped piece of equipment that rises from the top of the train to the wires.
KiwiRail said it snagged while the train was moving and it did so at one of the most critical and complex parts of the wiring system at the approach to the station.
The accident left the train disabled and the wires significantly damaged.
A company official said the reason for the snagging needed further investigation, and six KiwiRail staff would investigate.
The train operating company, Transdev, would also investigate.
KiwiRail said the damaged line had been restored, but the company was looking for long term options to make it more robust.
It said the accident happened in a place with a mix of older wiring and newer wiring which used different methods of maintaining wire tension.
KiwiRail was voted $98m last year to bring all of the Wellington Network, including the section damaged on Tuesday, under a modern system for keeping electric wires taut.
"In the interim we have put in some extra support for the wires," the official said.
"We will also endeavour to install some additional temporary poles or similar structures in this area to reduce the risk of the fixed-tension wiring sagging."
Once the investigation was concluded, further talks between KiwiRail, Greater Wellington Regional Council, and Transdev would decide on what to do next.