That article does not state that leaves can derail trains. The gunk/mulch that results from trains crushing leaves lubricates the wheels, which reduces the friction. That means it takes longer for the train to accelerate and brake. It can potentially be unable to stop as quickly as it should, which can potentially lead to more accidents, and then to more derailments. It contributes, it isn’t a cause.

It just reduces the wheel’s grip on the tracks a bit, not unlike how water on the road can reduce the grip of your tires. It’s still on the track, they’re not slipping off it like a cartoon stepping on a banana peel.