What action must be taken on both trains prior to resetting the emergency brake application?

The action that must be taken on both trains prior to resetting the emergency brake application is to apply the park brakes. This is crucial because it ensures that the trains are secured in place before any adjustments or resets are made to the braking system. Applying park brakes prevents any unintended movement that could occur during the reset process, maintaining safety protocols.

In the context of train operations, ensuring stability is paramount when dealing with emergency procedures. The park brakes act as a secondary safety measure, safeguarding against the possibility of the train rolling away while the emergency systems are being reset or evaluated.

Other options, such as increasing speed or releasing emergency brakes, do not align with standard safety practices. Resetting power would not be necessary as a prerequisite for resetting the emergency brake application, making the application of park brakes the correct and safest action to take.