Transport safety authorities have launched an investigation into how two runaway locomotives made their way on to a main freight line in Sydney's west last week.

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.