What must Dispatchers do regarding notifying Engineers after a weed sprayer has operated on the tracks?

When a weed sprayer has operated on the tracks, it is crucial for dispatchers to ensure that the appropriate personnel are informed about potential hazards. In this particular case, notifying the first three engineers is necessary to maintain safety and operational effectiveness. This group typically includes those who are most likely to encounter the affected segments of track shortly after the treatment has been applied.

By informing the first three engineers, dispatchers help ensure that these engineers are alerted to any special conditions or operational changes that may affect their train operations, such as reduced traction due to chemicals or any other operational implications resulting from the weed sprayer's activity. This proactive communication helps prevent accidents and maintains the safety of both train operations and personnel.

The other options, while they may seem plausible, do not follow the established protocol of notifying the first three engineers, which is a part of maintaining operational safety and efficiency in environments where herbicides or similar substances are used near the tracks.