On November 5, BHP was forced to derail a loaded iron ore train on Monday morning after it travelled 92km without a driver.
The train was travelling from Newman to Port Hedland when it stopped to allow the driver to inspect an issue with a wagon. While the driver was outside the train, it took off with no one on board.
BHP Western Australian Iron Ore asset president Edgar Basto said the company had completed its preliminary investigation.
"Our initial findings show the train came to a stop after a braking system control cable became disconnected," he said.
"The train began to move after the driver had disembarked to carry out an inspection, becoming what is termed a ‘rollaway' train."
Basto said the train was then derailed intentionally, because it could not be brought to a stop with the braking system.
"This decision was made with safety as the highest priority. The driver was unharmed and no-one sustained any injuries as a result of the incident," he said.
The preliminary investigation sought to understand why the incident occurred, and why it ended with the train having to be derailed.
"Our initial findings show that the emergency air brake for the entire train was not engaged as required by the relevant operating procedure," Basto said.
"In addition, the electric braking system that initially stopped the train, automatically released after one hour while the driver was still outside.
"Due to integration failure of the backup braking system, it was not able to deploy successfully."
BHP has put in a range of safety controls as a result of the early findings.
"Following this, we safely restarted our rail operations. Regulatory investigations are ongoing and we are working with regulators to learn from this incident," he said.
"Our focus remains on the safety of our people and our operations."