Final answer:
The exact speed at which dynamic brakes start to function is not provided in the reference information; it varies based on the train model and system design.
Step-by-step explanation:
The question pertains to the use of dynamic brakes on a train, which is a system typically employed to slow down the vehicle by converting the kinetic energy into another form of energy, like electrical or thermal, which is then dissipated. Unfortunately, the speed at which dynamic brakes start to function is not clearly provided in the reference information. Therefore, it is impossible to give an exact speed at which the dynamic brakes of a train will begin to function without more context or specifics from the question. Normally, the speed at which dynamic brakes engage can vary based on the train model and the braking system design.
The reference information provided does cover various aspects of vehicle dynamics, such as acceleration, deceleration, and recording the distance or time it takes for a vehicle to stop or reach a certain speed. In one example, a light-rail commuter train accelerates at a rate of 1.35 m/s² until it reaches 80.0 km/h and then decelerates at a rate of 1.65 m/s² until it comes to a stop. However, these examples do not explicitly mention the engagement speed of dynamic brakes.