Passengers travelling Muswellbrook and Werris Creek will need to take a rail replacement bus until the track can be cleared.

According to Transport for NSW, there is no timeline for when ordinary service will return

The site must be preserved until rail safety officers and investigators from the Office of the National Rail Safety Regulator can arrive at the site and undertake an investigation.

At midday, a spokesperson for the regulator told the Leader that their staff were "en route to the site".

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"A thorough investigation will continue over the coming weeks and focus on working with the appropriate parties to ensure the cause is fully understood and establish if any safety improvements are required to prevent a recurrence," the spokesperson said.

A freight train carrying grain operated by the Australian Rail Track Corporation (ARTC) derailed at about 7am on Thursday.

A spokesperson for ARTC said services are currently suspended on the Hunter Valley network between Werris Creek and Bells Gate.

"Planning for the recovery of services will take place when investigations are complete. There is no current timeline for a resumption of services," he said.

"ARTC thanks our customers and the community for their patience while we work to restore the line".

A spokesperson for NSW Ambulance told the Leader that the service had dispatched two vehicles to the crash, and treated two patients, both suffering minor injuries.