If the train has a 90 psi regulating valve setting, what is the minimum brake pipe pressure needed on the rear car before passing the summit of a mountain grade?

To safely navigate the summit of a mountain grade with a train that has a 90 psi regulating valve setting, a minimum brake pipe pressure of 75 psi is required on the rear car. This pressure is critical as it ensures that there is enough brake force available to manage the train’s momentum while descending the grade.

The regulating valve setting indicates the maximum brake pipe pressure the system will maintain, providing a baseline for understanding the operation of the air brake system. A pressure of 75 psi ensures that there is still sufficient braking capability even when the most significant forces are exerted on the train while going downhill.

If the brake pipe pressure were set lower than this, it might not provide adequate braking force, leading to potential safety risks. Conversely, higher pressures, while initially seeming beneficial, could lead to complications in operational effectiveness or maintenance of the brake system. Thus, the choice of 75 psi aligns with safety standards for train handling on grades.