Which of the following situations would require a train operator to immediately reduce speed?

A train operator must immediately reduce speed upon seeing a stop signal because this is a direct indication that the train must halt to ensure the safety of the passengers and the operation of the rail system. Stop signals are critical for maintaining safe distances between trains and preventing collisions. When a train operator observes a stop signal, swift action is necessary to reduce speed in order to comply with the signal's directive.

In contrast, while transitioning onto a different track or encountering a switch yard may require caution and potentially a speed reduction, these situations do not mandate an immediate reduction of speed like a stop signal does. Additionally, responding to a mechanical warning, while important, may not always necessitate an immediate speed reduction; it depends on the nature of the warning. Therefore, seeing a stop signal is unequivocally the situation that requires urgent action to reduce speed.