It says it "derailed" but that looks more like "it slammed through a closed emergency door at high speed, fell over, and derailed" which is a completely different type of accident.
What happened?
What happened?
The tunnel has several turnouts with doors between which allows them to send a train down the other tunnel when there is the need for maintenance or in case of an emergency. The door also seals the air which is part of the air evacuation system in case of a fire and also to allow the air to be moved in the direction of the train while in the other tunnel the airs moves int opposite direction.