In what situation do you need to make a 20 psi brake pipe reduction when cutting in air?

Making a 20 psi brake pipe reduction when cutting in air is necessary whenever a car is set out. This process helps ensure that the air brake system functions appropriately and maintains proper brake pressure throughout the train. When a car is removed from the train, the remaining cars need to ensure that they continue to operate safely and effectively with the existing air pressure. By making this reduction, it allows the remaining cars to equalize and adjust to the change in the system, thereby maintaining effective braking capabilities.

The other situations listed typically do not require a 20 psi reduction. For instance, during an emergency brake application, the air brake system automatically responds to a significant loss in pressure, making an additional manual reduction unnecessary. Replacing a drawbar also doesn't necessitate a similar reduction, as the air brake system's function remains unchanged during such maintenance. Finally, while it might be important to conduct certain procedures before opening the angle cock after setting out, the specific requirement of a 20 psi reduction is only applicable when a car is set out to ensure proper air pressure management in the rest of the train.