A disturbing incident on a San Francisco Muni train has raised new concerns about operator fatigue. Passengers were rattled when the light-rail car began jolting unexpectedly as the driver appeared to lose focus inside the Sunset Tunnel.

According to surveillance footage, the operator seemed drowsy moments before the train started swaying sharply, her head dropping several times as she fought to stay awake. The transit agency is now investigating the incident and reviewing its safety procedures to prevent a repeat.