What must not block the view of a blue signal?

The correct response highlights an important safety principle regarding blue signals. Blue signals are used to indicate that workers are present on or about the track and that trains should not move through that area. In this context, it is crucial that nothing obstructs the visibility of the blue signal to ensure that engineers and others operating trains or equipment are fully aware of the signal's presence.

When the view of a blue signal is blocked by equipment placed on the same track, it poses a significant risk because it may lead to a situation where a train inadvertently moves into an area where workers are present. Having an unobstructed view of blue signals is critical for maintaining safety in rail operations, allowing all personnel to be informed of where work crews are located.

Other options, such as the presence of other blue signals, operational lights, or train cars in the yard, may create confusion or signal complexity but do not directly interfere with the essential requirement that blue signals be visible to ensure the safety of workers. Therefore, ensuring that equipment on the same track does not block the view reflects the priority of maintaining safety protocols in rail operations.