When does the operating mode change upon entering yard territory?

The correct answer highlights that the operating mode changes as soon as the front of the train enters yard territory. This is significant because the front of the train represents the leading edge of the train's movement, and it is crucial for determining the overall operational status and safety measures that must be implemented when transitioning into a more controlled environment, such as a yard.

When the front enters yard territory, it indicates that the train is beginning a transition to a lower-speed operating mode, which is typically used for maneuvers and operations that involve switching movements, positioning, or coupling. In yard operations, safety protocols change, which may include reduced speed limits and increased awareness of potentially moving equipment and personnel.

The other choices are relevant considerations in terms of train movements, but they do not accurately reflect when the established operational mode is recognized or enforced in yard territory. Understanding this fundamental aspect is essential for train engineers as it ensures compliance with safety regulations and operational protocols that govern train movements in different territories.