If the Engineer applied the Automatic Brake, what would the control valve on a freight car sense in terms of brake pipe pressure?

When the Engineer applies the Automatic Brake, the control valve on a freight car senses a decrease in brake pipe pressure. This happens because, in an automatic braking system, applying the brakes causes the brake pipe pressure to drop, which communicates to the freight car's brake control system that the brakes need to engage.

This drop in pressure is the primary mechanism by which the brakes are activated across multiple cars in a train. As pressure falls in the brake pipe, the control valve responds by initiating the brake application on the freight car, ensuring that all connected cars respond uniformly to the braking action. This is crucial for maintaining control and safety during the operation of the train.

In summary, the decrease in brake pipe pressure indicates that the brakes are being applied, which is why that choice is the correct answer.