Ultrasonic testing showed an inconsistency in a section of rail which later broke, derailing a train north of Adelaide last year, but the finding was at the time thought to be due to the poor surface condition of the railhead, a new report has found.
The Australian Transport Safety Bureau this week released its final report into the derailment, which occurred just after 6am on July 28, 2017.
The driver of a Bowmans Intermodal containerised ore train, travelling empty from Port Flat, halted the train as he felt its performance was ‘very sluggish’ since rounding a curve on the approach to Dry Creek South.
Upon inspection, the train’s crew found the rear three wagons of the train had derailed.
There were no injuries reported, but there was substantial damage to the wagons, the track, and signalling infrastructure.
During its investigation the Bureau found the rail had broken during the passage of the previous train, after it developed a vertical split head defect due to “imperfections introduced during the manufacture of the rail 90 years ago”.
“The defect propagated vertically and longitudinally, roughly through the centre line of the lower leg rail in the curve approaching Dry Creek South,” the report states.
The rail brake affected the electrical continuity of the associated track circuit, preventing a signal from clearing for the next train – the one which eventually derailed.
According to the report, the network control officer authorised the train’s driver to pass the signal at stop, with the condition that the train travel at low speed.
The break itself was not visually evident to the train’s crew, according to the report. Once the back of the train rolled over the section, a two-metre section of rail fragmented, causing the last three wagons to derail.
ATSB investigators found ultrasonic inspection of the track roughly a month prior to the derailment recorded a sustained loss of back wall echo at the derailment location, automatically marking it with a spray of paint.
However, the ultrasonic inspection operator attributed the recorded event to the poor surface condition of the railhead, “which,” the Bureau said, “is a common condition that can inhibit the testing”.
“There was no retesting initiated or surface condition report lodged in response to the recorded event,” the report states.
“The absence of any follow-up missed an opportunity to identify the presence of the vertical split head defect prior to the rail fracture and the subsequent derailment of 1501S.”
Since the incident, the ultrasonic testing operator in question has reviewed the testing techniques used by operators where poor surface conditions exist, as well as the procedures for reporting and testing of rail affected by surface condition.
“Defects can develop in rails (and welds) due to a wide range of reasons,” the Bureau concluded.
“While poor surface condition of the railhead is a known limit to the effectiveness of ultrasonic testing, its presence can mask internal track defects, particularly when the condition exists over an extended area.
“If an inspection cannot test or can only partly test rails, maintenance personnel must report the shortfall to highlight operational risk and the requirement for a timely supplementary examination.”