Dynamic braking is particularly beneficial when operating in mountainous regions; where there is always the danger of a runaway due to overheated friction brakes during descent. In such cases, dynamic brakes are usually applied in conjunction with the air brakes, the combined effect being referred to as blended braking. The use of blended braking can also assist in keeping the slack in a long train stretched as it crests a grade, helping to prevent a "run-in", an abrupt bunching of train slack that can cause a derailment. Blended braking is also commonly used with commuter trains to reduce wear and tear on the mechanical brakes that is a natural result of the numerous stops such trains typically make during a run.