Runaway Hunter coal train in the Upper Hunter

The national rail safety regulator is investigating how a runaway coal train travelled for several kilometres west of Singleton early on Saturday morning.

The fully-laden and unmanned Pacific National locomotive travelled downhill for about two kilometres before coming to a stop at Nundah Bank at about 1am.

The incident happened during a crew change.

The rail corridor was temporarily closed to allow investigators to gather evidence and undertake an assessment of the circumstances surrounding the incident. Normal services have resumed.

"There were no injuries to our crew or any members of the public, and there was no collision or derailment," a Pacific National spokesman said.

Pacific National notified the Office of the National Rail Safety Regulator and Australian Rail Track Corporation network control.

The Office of the National Rail Safety Regulator said in a statement it was investigating the incident.

Transport advocates have called on Pacific National to introduce additional operational requirements, including ensuring that train crew members do not exit locomotives or trains in operational environments without another authorised person present and on board where practicable.

"Operators shouldn't have to rely on luck when it comes to the safe operation of heavy rail. Ensuring workers, in such a freight-heavy region as the Hunter, have appropriate support and backup when performing safety-critical duties should be a priority across the industry," Sylvia Duong, head of the North Sydney Transport Association Commission into the Newcastle Region said.