That doesn't mean it was the brake. The bearing contains the brake. The axle contains the bearing. The train contains the axle.

Not enough information to determine what caused the bearing to fail, bearing failures account for 5.9% of all train derailments[1].

Though we can look at common train bearing failures to get a likelihood...

Most likely it was a simple lack of maintenance. It's possible to prevent this with more sensors[2] and tighter maintenance checks.

[1] https://www.wisnerbaum.com/blog/2016/september/the-most-comm...

[2] https://www.southampton.ac.uk/engineering/research/projects/...