When the train brakes are applied during normal operation, the engine operator makes a "service application" or a "service rate reduction”, which means that the brake pipe pressure reduces at a controlled rate. It takes several seconds for the brake pipe pressure to reduce and consequently takes several seconds for the brakes to apply throughout the train. Pressure changes during a service reduction propagate at the local speed of sound, which means the brakes of the rear-most cars will apply sometime after those of the forward-most cars apply. Hence some slack run-in can be expected. The gradual reduction in brake pipe pressure will mitigate this effect.