The breathless local news (who do not know their local area very well) are speculating that a truck was on the tracks. Knowing that overpass, it is not hard to imagine. Easy access if it was indeed a non train vehicle.
By looking at the Google map there’s no way a truck could have been on the tracks without intentionally driving down the track.
There was no truck on the tracks, the tracks are on a bridge...misinformation.
Here’s the deal though, this bridge is important in one sense.
The line is going straight south until this bridge crosses i-5, so there is a fairly sharp turn as the rails go from due south, turning sharply S.E.
The speed along with the fairly sharp turn is the most likely reason for a derailment.
They have been testing this new line for the last couple weeks in a big way. The project to create this line has been huge, and the new stretch here has been worked on days and nights for over a year.