What action must be taken regarding the air brakes before securing a train at McPherson?

Before securing a train at McPherson, it is essential to release all air brakes. This action is critical to ensure that the train is not inadvertently held in place by the braking system when not in operation. Releasing the brakes allows the train to settle properly in its final resting position, which is crucial for both safety and maintenance.

In addition, releasing the air brakes helps in reducing any potential issues with brake stickiness or malfunctions that might arise if they are left applied for extended periods. By ensuring that the air brakes are fully released, it also allows for easier movement should the train need to be repositioned later.

The other options represent actions that could lead to safety or operational issues. For instance, applying all air brakes would prevent the train from being safely secured for downtime. Testing the air brakes, while important during other operational phases, would not be the necessary step when preparing the train for a static state. Leaving air brakes unchanged could potentially lead to unwanted train movement or complications when securing the train.