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.
High speed for that set up would be about 80mph max, and definitely not that fast through that stretch of track. Definitely some speed restrictions going through there. So it could be operator error, not following speed restrictions, equipment malfunction, track problem, collision, or sabotage.
We won’t know until the investigators interview the engineer, and review the event recorder data and video downloads.