"Microsleeps" were the likely cause of an incident near Pongakawa last year when a freight train made unauthorised entry into a work site.

A report, released by the Transport Accident Investigation Commission, showed the driver had missed warning and compulsory stop boards leading up to the work site and crossed a bridge while workers were still under it.

The report showed no injuries or damage to the train or bridge occurred.

About 10am on February 7, 2017, the freight train was en route from Mount Maunganui to Kawerau.

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The report showed the driver was nearing the end of a 10.5 hour night shift.

Later, the driver did not recall seeing any of the warning or compulsory stop boards and only noticed the presence of a hi-rail maintenance vehicle beside the track as the train was approaching the bridge.

The driver reduced the train speed to 25km/h shortly before crossing the bridge without authorisation.

The Transport Accident Investigation Commission found it likely that the driver was experiencing "microsleeps" at the time the train passed the warning and stop boards, and the driver was later diagnosed as suffering from a sleep disorder that affects the quality of sleep.

"Additionally, the driver had had difficulty sleeping the evening prior to the incident due to the hot ambient temperature," the report said.

The fact the driver had been awake for more than 10 hours through the night and was nearing the end of the shift was also a factor.

KiwiRail now has a system to investigate and resolve potential faults in safety systems and the Transport Accident Investigation Commission recommendations address medical data capture; sleep apnoea detection; and the need for a good fatigue risk management system.

Key lessons arising from the inquiry were:
- Train drivers and other shift workers need to ensure that they are medically fit and make appropriate lifestyle choices that will enhance the amount and quality of their sleep, in order to avoid being fatigued or tired while at work
- Transport operators must ensure that their staff are fully educated on the factors that can cause or contribute to their becoming tired or fatigued while performing safety-critical roles
- Technological systems need to be fully tested and have undergone full failure mode analysis if they are going to be relied on as safety defences for preventing accidents and incidents.