Rail Bus and Tram Union central branch organiser Bruce Mackie told the Daily Mercury he understood that heat had buckled the tracks.
"There was extreme heat on Friday and our members are reporting it appears the track buckled from it," Mackie told the newspaper.
A published photo of the aftermath showed at least five empty wagons off the rail, with one sideways stacked on top of another, and at least three separated pairs of rolling stock wheels.
Aurizon said it worked with other rail operators, clients and supply chain partners to minimise service disruptions.
An investigation is continuing.