The driver was on the train when it left Newman yesterday morning at 4:40am but according to BHP the worker jumped out to inspect a wagon near the Hester siding, but then could not get back on.
The train, which comprised four locomotives and 268 wagons, travelled 92km in 50 minutes at speeds of around 110km/hr before BHP was forced to derail the runaway at a set of points 120km from the port.
The Australian Transport Safety Bureau is investigating the incident, which it said caused significant damage to the train.
A BHP spokeswoman said no-one was injured in the derailment, and it has suspended all train operations while the investigation is carried out.
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