Commuter Tony Amar's photo of the damage caused when the air compressor used in the train's brakes was shot up into the floor of the passenger carriage. Amar was thrown into the air and believes that, if he had been standing in the aisle, he would not have survived.

A Wellington train derailed by faulty brakes left an eight-kilometre trail of broken machinery behind it before coming to a shuddering stop.

A Transport Accident Investigation Commission report published on Thursday found a "safety-critical" part of the brakes were left unsecured for more than 70 days before the derailment, near Wellington in May 2013.

The part eventually fell off during the morning commute from Porirua into Wellington on May 20, wedging into the belly of the train carrying 315 people, causing it to derail, and blasting a hole in the passenger carriage floor.

CAMERON BURNELL/ FAIRFAX NZ Tony Amar is annoyed that KiwiRail has never apologised to him or other passengers caught up in the accident.

Four people suffered minor injuries in the crash, and the entire fleet of ageing Ganz Mavag trains was pulled off the rails to undergo safety checks.

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The report has found that maintenance staff failed to install a split pin that bolted the brake component to the rest of the train when they replaced the brakes. In the 10 weeks between then and the derailment, thousands of commuters travelled on the train.

THE TRANSPORT ACCIDENT INVESTIGATION COMMISSION When part of the brakes broke free of the train and fell onto the track it caused extensive damage. These air tanks were "jack-knifed", breaking free from their mount and twisting, causing the air compress to shot into the floor of the passenger compartment.

"Without anything to secure the bolts in place, they worked loose over the following 10 weeks in service and eventually dropped out," the commission said.

It found maintenance of the trains relied too heavily on engineers' know-how, with few check or records of "safety-critical" work.

"The braking system on a train is a safety-critical system, as are its components. A simple check by an appropriate person ... would have prevented this accident."

FAIRFAX NZ A report has found the Porirua to Wellington commuter train, pictured, derailed after a brake component that was not properly secured fell on the track.

In 2013, the commission also recommended "urgent" changes to how safety-critical parts of the train were maintained.

KiwiRail general manager metro David Shepherd said that, since the derailment, maintenance had been overhauled in line with the commission's recommendations, including independent checks of all safety-critical work.

The three workers involved in failing to secure the brake component were all still employed at KiwiRail and had not faced penalties. It was the system, rather than any one worker, that was to blame, Shepherd said.

"At the time it was pretty distressing for staff. It's not what they come to work to do."

The commission report showed brakes started falling apart at least 8km before the train eventually derailed, with investigators following a "debris trail" back along the tracks.

Passengers as far back as Porirua noticed "scraping noises" coming from beneath the rear carriage, probably the sound of the unsecured brake part being pushed towards the rails as the driver hit the brakes.

When the train did stop, about 2km from Wellington railway station, witnesses described a loud bang and a three-metre-high stream of vapour rising from beneath the rear passenger carriage.

Afterwards, passengers described their terror as the floor was ripped open and they were thrown about the carriage.

"There were passengers screaming. When we came to rest, we [were] skewed from our neighbouring carriages sideways, about half a metre, " passenger Zoe Glasson said.

Shepherd said the crew would not have heard the screeching, but such a derailment would not occur again with the stricter maintenance regime in place. "We are in better position to help catch any errors that may occur during maintenance work."

"SMOKE AND DUST"

Tony Amar is still bitter more than two years after he was thrown from his seat, as the train's brakes fell apart beneath his feet.

"There was a bang and smoke and dust, people were yelling and screaming," he said. "Afterwards everyone just wanted to get out but we couldn't."

Amar was one of four people injured when the carriage he was in derailed. He banged his shoulders and neck as he was thrown about.

When the train stopped, he found he was sitting next to a gaping hole, where the brakes air compressor had shot like a rocket through the floorboards.

"KiwiRail was very fortunate that no one was standing there at the time because it could have been fatal," he said.

He remained "deeply disappointed" he had never received an apology from KiwiRail for the "nightmarish" ride. Despite everyone in the carriage giving their details at the time, he had heard nothing from the company since.

"I'm disappointed they didn't check everyone was OK. They could have at least said they were sorry."

KiwiRail general manager metro David Shepherd said police had taken passengers' details, not KiwiRail. The company later tried to obtain this information, without success.

"One of the learnings for KiwiRail was that we should have collected passenger details to assess their well-being," he said.

Staff also received 36 compliments from passengers whom they helped after the derailment, he said.

While he bikes more these days, Amar still regularly commutes by train from his home in Tawa. "Every time I get on, I wonder whether it's the same train."

WHAT WENT WRONG

On March 7 the Ganz Mavag carriage went in to have the spring park brake replaced.

When the shift ended, the mechanical engineer involved went on annual leave, so sent a handover email to the next day's shift, outlining what still needed to be completed, including the line "All that needs to be done is – lock wire bottom bolts – release split pin and test for correct function".

The next day, the train was released for service after being signed off. There was a verbal confirmation that the work was complete, but no physical inspection.

Split pins secure the top hanger pins, which take the weight of the spring park brake. Without the split pins, the top hanger pins worked loose, reducing the support for the brake which, on May 20, dropped down to below track height between Porirua and Wellington.

While travelling just under 60kmh, the train's brake pressure suddenly dropped and the train braked automatically, stopping 17 seconds later, 151 metres down the track.

The rear coach derailed, and an air compressor was forced up through the carriage floor.