A toxic gas alarm sounded after a TranzAlpine train carrying 177 passengers broke down inside a remote Canterbury railway tunnel, leaving everyone stranded for more than an hour.

A new report from the Transport Accident Investigation Commission has revealed the circumstances surrounding the breakdown on 30 May 2026, including earlier electrical faults and a mechanical failure that brought the train to a standstill.

The Christchurch to Greymouth service was travelling through an isolated section of the Midland Line when it lost power shortly before 9.45am.

The train continued slowing before stopping inside Tunnel 9 at 9.47am, with all 10 carriages, six crew members and the locomotive engineer inside.

Its front was approximately 100 metres from the western entrance, while the rear remained about 250 metres from the eastern end.

The driver applied the brakes, made an emergency radio call to train control and began shutting down the locomotive to reduce diesel fumes.

During that process, the driver’s gas monitor sounded an alarm indicating increased levels of toxic gases in the air.

The report does not identify which gases were detected, their concentrations or whether passengers were exposed to dangerous levels.

The location presented difficulties for the recovery operation, with the railway passing through the Waimakariri and Broken River gorges, where there is no road access to the tracks.

KiwiRail declared a Level 3 emergency and began arranging assistance, directing locomotives from a freight train at Springfield towards the stranded service.

However, at 11.02am, the driver managed to restart the locomotive.

With permission from train control, the train moved out of Tunnel 9 and continued through another tunnel before stopping at a safer location.

Passengers had spent approximately 76 minutes inside Tunnel 9.

Recovery locomotives arrived at 11.45am and were coupled to the disabled train, allowing it to be hauled back towards Christchurch.

Passengers were able to disembark at Springfield and Rolleston, with the remaining travellers arriving at Christchurch railway station at 2.23pm.

Nobody was injured.

Investigators have established that the breakdown resulted from a failure in the mechanism responsible for operating the locomotive’s cooling fan.

The fan was designed to supply air to important electrical equipment, including the traction motors, rectifiers and batteries.

When the gears driving the fan failed, the equipment overheated, triggering a protective system that cut power to prevent further damage or a possible electrical fire.

The commission also discovered the locomotive had experienced two earlier electrical faults while travelling between Rolleston and Springfield.

These occurred at 8.45am and 8.47am, but both reset automatically, allowing the journey to continue.

A further fault developed at 9.44am, when the locomotive was travelling at 30 kilometres an hour. The train subsequently lost the power needed to keep moving.

The locomotive involved, DXC 5391, was manufactured in 1976 and had been hauling a 380 tonne passenger service measuring 208 metres in length.

The commission confirmed the driver had been employed by KiwiRail since 2012, began driving locomotives in 2014 and had been certified to operate passenger trains since September 2025.

Investigators inspected the train, interviewed staff and examined maintenance records, operating data and communications with train control.

Although the immediate cause of the breakdown has been identified, the wider investigation remains unfinished.

The commission said it was continuing to verify evidence surrounding the mechanical failure while pursuing several lines of inquiry into wider systemic matters.

Its final report will contain findings, identify any safety issues and make recommendations where necessary.