Sensor indicated a wheel bearing was overheating, over 250 degrees above normal. They tried to slow the train down, but there is
no info on when the sensor indicated there was a problem, when they tried to slow down, and when the train derailed. Train wasn't speeding.
article said:
As the engineer applied the brakes, an automatic braking system kicked in, according to
the report. Investigators found that a wheel bearing was heating up over several miles as the train approached the derailment site, according to data from trackside sensors, but did not reach a critical threshold until shortly before the incident, when it registered 253 degrees above normal.
So, it looks like this will be a live and learn thing and Congress might (won't) require better monitoring. Obviously, avoiding derails is in a RR's interest, but monitoring every car, that has to be pricey.