Samuel F. Belcher’s colleagues called it in over their radios when his behavior caused delays during a late-night shift: The train conductor was “acting funny,” they said, staggering and showing signs of confusion at a Hamlet, North Carolina, station in January 2012.

Minutes later, when Belcher was supposed to send the cars off the main track toward the maintenance yard, the experienced conductor instead misaligned two switches, causing the train to derail.