The train derailed on the Sandy Hollow-Gulgong rail line at Baerami, near Muswellbrook, early on Tuesday morning.
"At approximately 0325 AEDT, Pacific National coal train MR280 travelling towards Newcastle derailed as it was travelling through a section of the [Australian Rail Track Corporation] network that was under a temporary speed restriction of 20km/h," the ATSB said in the summary.
A track engineering report, locomotive data-logger information and train crew interviews will form part of the ATSB investigation. A final report will be released later this year.