Commuter Train Disaster In Los Angeles

On Friday in the outskirts of Los Angeles County a commuter train and a freight train had a head on collision. For the last 16 hours over 200 fire fighters and rescue people have been cutting people out of the wreckage, some alive some dead. This is the worst train accident in the history of Metrolink commuter train. What caused the two trains to be on the same track at the same time traveling towards each other is still unknown?