Very tragic to lose three lives.
The September 2021 Trains Magazine has an excellent article on broken rails "The Science of Broken Rails". It notes that "broken rails are a leading cause of derailments".... and "the overwhelming majority are found and repaired before a derailment occurs." The article also notes that "complicating the derailment investigation, is the fact that the point of derailment is often obscured by the pile of derailed cars."
So, given that this was a passenger train with a dozen or so cars and not a freight train with over 100 cars, if a broken rail was the cause, it would be found easier/quicker. Some of the challenges is to find the initial broken rail, since there are probably a lot of rails broken after the crash started. However, it was noted that the majority of broken rails occur during the winter as the cold weather causes steel to contract.
A distant second cause of derailments are various events like: journal/roller bearing failure, wide gauge, or buckled track.
It was an interesting article, and included technology being employed to detecting rail flaws.
I have always wanted to go on the Empire Builder, this derailment is not going to derail my dream to do this trip and enjoy the sights.
