The derailment could have been caused by a vehicle which that from a slope near the tunnel, and hit the moving train, according to CNA.

A video on social media showed parts of an industrial vehicle heavy equipment, along with what looks like a car door beside the derailed train. In the video, a man can be heard saying, “Our train hit the truck. Where is this place? Qingshui Tunnel, the accident happened at Qingshui Tunnel. The train hit the truck, the truck fell off. It’s lucky that I was at the later carriage — I was at the fourth carriage.”

https://worldtopnewsng.com/48-confirmed-dead-dozens-injured-after-train-derails-in-taiwan/