According to the federal Office of the National Rail Safety Regulator (ONSR) the "primary cause" of the accident was incorrect train coupling and "actions taken by the driver after the train uncoupled."

Rail services were disrupted after a grain train derailed just outside the town, at about 7am on January 6. The driver suffered minor injuries in the crash.

The rail line was reopened a day later, after crash investigators had studied the site.

A spokesperson for operator Southern Shorthaul Rail said the circumstances that led up to the incident were "unprecedented in the 167 years of rail operations in NSW".

"An incident of this type has never occurred before," he said.

The company is amending its coupling procedure to ensure that locomotive power is used in every instance to ensure that rollingstock has been correctly coupled together, he said.

ONSR completed a report into the January 6 train derailment this month.

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According to a spokesperson for the regulator, the major cause of the crash was incorrect coupling of the train during the shunting process at Werris Creek and actions taken by the driver after the train uncoupled during its journey.

"ONRSR is now working with the operator to address its documented processes and procedures along with its training of personnel, making sure they have been extensively reviewed and amended to ensure such an event does not occur again," he said.

"As with all accredited rail transport operators, the operator in this case will be subject to ONRSR's ongoing program of audits and inspections of all operations."

An investigation by the Australian Transport Safety Bureau is still open. It is expected to be completed in the second quarter of this year.

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A preliminary summary by the bureau shows the crash occurred after banking locomotives were attached to the rear of the grain train, at Werris Creek, in preparation for banking operations up the heavy grade between Chillcotts Creek and Ardglen.

"Shortly after departure from Werris Creek, the three banking locomotives separated from the rear of the grain train," the summary reads.

"As a result, the grain train's brakes automatically applied and the train came to a stop. Shortly afterwards, the three banking locomotives collided with the rear of the stationary train, resulting in the rear wagon of the train lifting from its rear bogie and impacting with the [locomotive]."

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The freight train, operated by Southern Shorthaul Rail on the Australian Rail Track Corporation, was carting bulk grain from Narrabri to Bullock Island Graincorp.

Services were suspended on the Hunter Valley network between Werris Creek and Bells Gate as a result of the crash. Passengers were forced to take rail replacement buses for part of the journey to Newcastle and Sydney, and freight blocked.

A spokesperson for Southern Shorthaul Rail said that the all rollingstock involved is expected to be repaired and returned to traffic.

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