I read an article a little while ago that said PTC was installed on this track. This section was recently overhauled to the tune of $180 + million. Problem was, this was the first day to run the train on the track and they hadn't yet activated the PTC. The whole thing sounds like a major screw up.
https://www.seattletimes.com/seattle-news/amtrak-train-derailment-olympia-interstate-5-seattle/
Train orders for that curve are
30 mph, per the article above. If you look at the overhead photos of the accident, you can see it's a very sharp curve. A motorist on I-5 was quoted as saying he saw the train traveling parallel to the highway before it crashed. He said it passed was traveling faster than him and he was going 60 mph.
Conductor and engineer were disoriented, but they survived the crash. They should be able to participate in the investigation, JMO.
Update: Here's audio of the conductor calling for help on his radio. He says they just went off the track at the curve, no mention of any obstacle. I can't copy the link, but just scroll down the page at the Seattle Times