Jason Rea, chief engineer for the western region of Union Pacific Railroad, holds a lag screw, at least one of which is blamed for shearing off and causing the June 3 oil train derailment in Mosier.

Jason Rea, chief engineer for the western region of Union Pacific Railroad, holds a lag screw, at least one of which is blamed for shearing off and causing the June 3 oil train derailment in Mosier.

Neita Cecil/The Dalles Chronicle

The Mosier train derailment was caused when an unknown number of large screws, used to provide extra stabilization to rail ties on curves, sheared off — something a railroad official said he’d never seen before in a derailment.

Jason Rea, chief engineer for the western region of Union Pacific Railroad, described at a community meeting Friday in Mosier what had caused the June 3 derailment of 16 oil cars.

The so-called lag screws, which are threaded, are used on curves instead of a straight track spike. And while the lag screws had been severed about two and a half inches below the head of the screw, the top of the screw did not dislodge. Had they dislodged, it would have been detected by a visible inspection, Rea said.

Rather, the sheared screw or screws remained in place.

Read more at The Dalles Chronicle.