Quick service locally vents brake pipe pressure by how much?

Quick service locally vents brake pipe pressure by 4-6 psi because this amount allows for a rapid but controlled decrease in pressure, which initiates the application of the brakes effectively without causing a harsh or abrupt impact on train stability. This range is specifically designed to ensure that brake systems respond quickly to the engineer's commands while also preventing excessive loss of pressure that could lead to complications in train handling or negatively impact the performance of the braking system.

In contrast, larger pressure reductions might lead to more severe braking, which could be inappropriate in certain scenarios, potentially resulting in track and train safety issues. This careful calibration is crucial in maintaining reasonable control over train dynamics during various operating conditions.