Before passing the summit of a mountain grade, what should the rear car brake pipe pressure be within?

Before passing the summit of a mountain grade, it is essential that the rear car brake pipe pressure is maintained within 15 psi of the regulating valve setting. This range ensures that the air brake system is functioning optimally, providing adequate braking power as the train descends. Maintaining pressure within this limit helps to prevent either insufficient braking or unintended brake applications that could occur if the pressure varies too much.

Having the pressure within the specified range allows for effective control of the train while descending, which is crucial for safety and efficiency in mountain-grade operations. If the pressure were to drop or rise too significantly beyond this range, it could lead to inadequate braking response or increased risk of damage to the train or track conditions. Keeping the brake pipe pressure stable within these parameters is a critical safety protocol that train crews must adhere to during mountain operations.