What is a necessary step if an emergency stop button is activated?

When an emergency stop button is activated, one of the critical actions that must be taken is to apply the park brakes immediately. This step is crucial for ensuring the safety of the equipment and personnel. By applying the park brakes, you prevent any accidental movement of the machinery or vehicles involved, stabilizing them and mitigating potential hazards that could arise during an emergency situation.

Applying the park brakes effectively locks the equipment in place, reducing the risk of collisions or uncontrolled movements that could lead to injury or further mechanical issues. It's a proactive measure to secure the systems quickly in response to the sudden need for an emergency stop.

In this context, other options would not prioritize safety in the same manner; continuing operations after checking could lead to dangerous situations if the incident has not been fully resolved. Allowing the trains to coast to a stop does not provide immediate stabilization, which is essential in an emergency. Additionally, powering on both trains doesn't address the urgent need to ensure safety following an emergency stop. Thus, applying park brakes is the necessary immediate action following the activation of an emergency stop button.