When should an empty train bound for the yard start entering a station occupied by a regular passenger train?

The correct answer emphasizes the necessity for the empty train to ensure safe passage through the station, which is critical when there is a regular passenger train present. It is important for the entire empty train to clear the station area, as allowing it to enter while only partially in the station could pose safety risks. This is particularly relevant if the two trains are scheduled to occupy the same station at the same time, as having a train partially in the station could hinder the movement of the passenger train or create hazards for passengers boarding or alighting.

Starting to enter only when it can clear the entire train ensures compliance with safety regulations and operational procedures, preventing any potential accidents caused by obstructions. This approach supports a more organized flow within the station and enhances overall safety for both trains and passengers.

In contrast, the other options suggest initiating the train's entry based on different conditions, such as the arrival of a green signal, the departure of the passenger train, or immediately upon arrival, which could lead to unsafe situations if the empty train cannot fully clear the station in time for the passenger train's operations.