What defines the speed that allows stopping within half the range of vision?

The term that defines the speed allowing a train to stop within half the range of vision is restricted speed. This concept is critical in train operations, as it ensures that a train can react to unexpected conditions or obstacles that may appear within the track's sight distance.

Restricted speed is typically used in situations where visibility is limited, such as in areas with track work, around curves, or in situations involving other trains or vehicles on the tracks. By operating at this speed, the train crew has the necessary reaction time to take corrective actions, such as applying the brakes, in order to safely come to a stop before reaching any potential hazards.

The other options relate to different speed concepts but do not specifically address the need for immediate stopping within half the range of sight. Normal operating speed refers to the standard speed for trains under normal conditions. Maximum safe speed is the highest speed a train can travel on a particular segment of track without risking safety. Minimum speed indicates the lowest speed at which a train can safely operate on certain segments of track. None of these adequately represent the precautionary aspect of stopping within half the range of vision as effectively as restricted speed does.