The rate at which the change in air pressure initiated by the operation of the driver's brake valve travels along the brake pipe. Because it takes time for the reduction (or increase) in pressure to travel along the brake pipe, the brake applies (or releases) at different times on different vehicles. This affects the control of the train and can cause bunching or stretching of vehicles putting additional strain on the couplings, particularly on long, heavy trains. Skill is required by the driver of a heavy freight train to ensure that operation of the brakes does not cause a train to break apart or even derail.