“grafitti on the bridge (LABRAT KHAT)”
?
https://www.nytimes.com/2017/12/18/us/amtrak-derailment-washington.html
The National Transportation Safety Board said at a Monday night briefing that the train had been traveling more than twice the speed limit before it derailed, or at 80 miles per hour instead of the allowable 30 m.p.h.
Bella Dinh-Zarr, a member of the safety board, said at the briefing that it was unclear why the train was traveling so fast or whether the operator’s unfamiliarity with the new route had caused the accident.
These letters are visible on the bridge that the train was attempting to cross. They were also there when google earth captured its images in Nov 2016. This is how I knew which bridge was involved. As to the meaning, no idea, some dude who is a lab rat and a “khat” — not necessarily any indication of bad intentions or certainly (not) any connection to this sad accident.