Neither Pacific National locomotive had a driver on board when they ran away from a freight terminal at Chullora and on to the rail line for a short distance, the Australian Transport Safety Bureau (ATSB) said.

Pacific National confirmed that three of its employees had been stood down pending an investigation into the incident, which occurred at the company's Sydney Freight Terminal about 5.50pm on Wednesday last week.

An anonymous caller to radio station 2UE claimed that the diesel locomotives, which did not have wagons attached to them, reached speeds of between 20km/h and 30km/h.

The locomotives were heading towards Flemington before they stopped on a steep incline in the track and rail crews caught up to them, the caller said.

"If another train was heading towards them, fatalities could have happened," the caller said.

An ATSB spokesman confirmed that it had launched an investigation into a "serious incident" in which two locomotives ran away.

He said no one was injured and there was no damage to rolling stock or fixed infrastructure.

"The ATSB has commenced an investigation into this serious incident, and two investigators from the ATSB's rail safety investigation team attended the Chullora yard on September 29," he said.

"The investigation will independently examine the circumstances surrounding the runaway, including the provisions for rolling-stock security and protection of the main line."

A Pacific National spokesman confirmed an incident at its Sydney freight terminal, in which two locomotives "left our yard and entered the mainline freight network".

"At no stage did the locomotives enter the Sydney Trains passenger network," the spokesman said.

"There were no injuries and we are currently working with ARTC [Australian Rail Track Corporation] and the rail safety regulator on an investigation into the incident.

"Three employees have been stood down pending the outcome of the investigation."

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