During an FRA Class II air brake test on PA5, what will happen to the brake cylinder when the engineer moves the controller to 'coast'?

When the engineer moves the controller to 'coast' during an FRA Class II air brake test on PA5, the brake cylinder will move to zero. This is because shifting to the 'coast' position indicates that the brakes are being released. In this state, the pressure in the brake cylinder decreases as the air is exhausted, leading to a state where the braking force is effectively neutralized, bringing the brake cylinder pressure down to zero.

Understanding this behavior is crucial for train operations, as it ensures that the train can move freely without the drag of the brakes engaged. In practical terms, this means that the brakes are not actively applying force to slow the train; therefore, the reduction in brake cylinder pressure is essential for achieving a 'coasting' condition, which allows for smooth transitions in speed without risk of unintended braking effects.