Just looked at it on google maps. Those two curves where the track crosses I-5 look way too sharp for fast rail service. Also, the ballast and grade look like they’ve been worked on recently.
But yeah, it looks like an old subdivision that’s been re-utilized.
I think people are ignorantly using the term “high-speed”. The engine is a normal Amtrak engine, not high speed. Compared to freight, yes. I seriously doubt they’d intend high speed on a branch which this seems to be.
The reports said the curves are 30mph curves, so the train was supposed to be pretty slow at that point if they are so close to the bridge.
Freight lines have sharper curves then passenger lines. Freight cars are shorter than passenger cars which allows them to trail over shorter radii rail curves.