What should the engineer do upon receiving an over speed warning?

When an engineer receives an over speed warning, the appropriate action to take is to slow down immediately. This response is crucial for maintaining safety within the operating environment. Over speed can lead to dangerous situations, including equipment failure, derailment, or increased wear on components, all of which can have serious consequences.

Slowing down allows the engineer to regain control of the situation and ensure that the operation remains within safe limits. It's a proactive approach to prevent potential hazards that could arise from excessive speed. Furthermore, immediate action to reduce speed can help mitigate risks and ensure compliance with safety regulations, which are designed to protect both crew and passengers.

The other options listed do not prioritize safety and could exacerbate the situation. For example, increasing speed (the first option) directly conflicts with the necessity to manage over speed warnings, putting the operation at greater risk. Checking the service brakes or notifying passengers, while potentially useful depending on the situation, should not take precedence over addressing the immediate threat indicated by the over speed warning. Consequently, slowing down is the most responsible and prudent response in this scenario.