How much PSI reduction is required for a Class I Brake Test to apply the brakes?

The requirement for a Class I Brake Test specifies that a reduction of 20 PSI in the brake pipe pressure is necessary to apply the brakes effectively. This is a standard practice to ensure that the brakes are functioning correctly and can respond in a timely manner. A 20 PSI reduction indicates that there is sufficient pressure change to activate the brake system, allowing for a smooth transition from a running status to a stopped position.

In the context of understanding train operations and safety protocols, it's crucial to know the specific PSI requirement because this knowledge directly influences how crew members perform brake tests and respond to emergency conditions. The other amounts of PSI mentioned do not meet the standardized requirement for a Class I Brake Test, which is specifically designed to ensure optimal safety and control during train operations.