What is the required action if operating conditions allow when preparing to depart?

When preparing to depart, increasing the brake pipe reduction to 15 psi is necessary because it ensures that the braking system is appropriately charged, aiding in establishing the required pressure differential for effective brake operation. This action helps to ensure that the entirety of the train's air brake system is properly activated and ready for use. It allows the engineer to manage the brakes effectively, ensuring that they can respond adequately during the operation.

While other actions are important in train handling and safety, such as releasing all brakes completely or conducting a safety briefing, they do not directly address the primary operational requirement of preparing the train’s braking system for a safe departure. It's important to prioritize the proper adjustment of brake systems to ensure the readiness of all equipment, which is why increasing the brake pipe reduction to the specified level is the most suitable choice in this context.