ARLINGTON, Texas (AP) — A loose, improperly fastened rail joint led to a 2017 derailment in a North Texas rail switching yard that killed a worker, the National Transportation Safety Board tweeted Tuesday.

A misaligned rail joint allowed the flange of a freight car wheel to climb the rail and derail in the Union Pacific Great Southwest switching yard in Arlington on the morning of Sept. 22, 2017, killing yard worker Jon Beckman, 48, of Frisco, the NTSB said in a report.

The track near the Texas Rangers ballpark was exempt from federal rail track safety standards, allowing inadequate track conditions to exist on the regularly used track, the NTSB reported last week.

The agency didn’t explain why the track was exempt.

A message to a Union Pacific spokesman seeking comment Tuesday was not immediately returned.

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