What should a train operator never do while the train is in motion?

A train operator must prioritize safety while the train is in motion, and using handheld devices is a critical action to avoid. Operating a train requires full attention and focus; engaging with handheld devices can lead to distractions that might impair the operator's ability to respond to operational needs or unexpected situations.

Centering the reverser is a standard procedure that is sometimes necessary for train operation and can be done without compromising safety, provided the operator is attentive to the surroundings. Checking the brake system is part of a train's operational checks and should typically be performed when the train is stationary; however, inspecting brake functionality while the train is moving can sometimes be achieved safely through existing systems. Adjusting the speedometer is not a typical task a train operator performs while the train is operational as the speed should be monitored through designated instruments without manual adjustments during motion, but it does not involve distraction as an inherent safety risk like handheld device usage.

Thus, utilizing handheld devices while the train is moving is the most critical action to avoid, as it poses a significant safety hazard by diverting the operator's attention from the essential task of operating and monitoring the train effectively.