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‘Lateral harmonic wagon oscillation’ caused 2014 derailment, ATSB says

iron ore derailment - Photo ATSB

The derailment of an Aurizon iron ore train in WA last year was initiated by “lateral harmonic vehicle oscillation,” the Australian Transport Safety Bureau said late last week.

On May 14, 2014, train 4413 – a bulk iron ore train – derailed on the Defined Interstate Rail Network (DIRN) between Stewart and Bonnie Vale, in WA.

Wagons and track were severely damaged during the incident.

A final report handed down on March 27 by the ATSB reported the believed cause of the derailment, and it’s a rather complex one.

The ATSB found:

…the derailment of train 4413 was most likely initiated by lateral harmonic vehicle oscillation induced by a combination of minor cyclic cross-level and lateral track irregularities just in advance of the point of derailment.

As a result of these irregularities, it was likely that the roll of wagon WOE33548K caused the left hand wheels to unload at a time when the leading left wheel came into contact with the left rail face – resulting in flange climb and derailment.

The wagon type that derailed, WOE class, had passed prescribed dynamic performance testing, and the track and wagons complied with mandated requirements, the bureau said.

But computer simulation undertaken after the derailment showed the lateral harmonic wagon oscillation was of sufficient magnitude to increase the likelihood of derailment at the site.

Simulations showed that iron ore wagons, with their short length, react more severely to 22m wavelength cyclic irregularities (as evident at this site) than do the typically longer intermodal wagons.

The ATSB said the area of track where the derailment took place is “known (to train drivers) as an area of rough ride.” But the bureau found that the system in place to report such track irregularities was “ineffective, and hence the opportunity was lost to check for uncharacteristic track qualities through the derailment site, before such qualities contributed to a derailment”.

As a result of the investigation, Brookfield Rail and Aurizon have developed enhanced procedures for reporting track irregularities, the bureau explained.

The pair have also jointly committed, through RISSB, to ongoing industry support and research into the cause of this type of derailment.