The Pacific National train was en route to Narrabri for loading when 19 empty wagons derailed just after 5am on February 15 at Kankool, near Willow Tree.

The Australian Transport and Safety Bureau (ATSB) is investigating the incident with officers from the NSW Office of Transport Safety Investigations.

The crew of the NB901 coal service contacted the Network Control Centre North at Broadmeadow, reporting their train had lost the air and come to a stop.

It was only when the assistant driver went to investigate the loss of air that he discovered the leading 19 wagons had derailed and come to rest in a cutting. 

No one was injured during the derailment but the track sustained severe damage during the incident.

The line was blocked for five days following the derailment.

A Senate Estimates hearing was last month told the Kankool incident was likely to cost the Australian Rail Track Corporation about $1 million.

Investigators have begun collecting evidence and statements from parties involved in the incident which will form part of a draft investigation report.

According to the ATSB, the draft report will be forwarded to the agencies involved for comment before the final report is released.

The ATSB anticipates the report will be released in November this year.