What should the regulating valve on the controlling locomotive be adjusted to for a freight train?

The regulating valve on the controlling locomotive for a freight train should be adjusted to 90 psi because this setting effectively balances the need for sufficient braking force with the operational requirements of the train. At 90 psi, the air brake system maintains optimal pressure to ensure reliable brake application across the entire length of the train, which is particularly important for heavy freight consist.

This pressure helps ensure that when the brakes are applied, there is enough braking force to control the speed of the train safely during both regular operations and emergency situations. Setting the valve too low could result in inadequate braking force, especially on downhill grades or during emergencies, while setting it too high could lead to excessive braking force, potentially causing wheel slip or other control issues. Therefore, the 90 psi setting represents a well-established industry standard for freight operations, balancing efficiency and safety.