On train movements requiring dynamic brakes, what additional equipment is needed besides an operative dynamic brake?

The correct choice is the requirement for an operative accelerometer displaying current speed changes when utilizing dynamic brakes on train movements. An accelerometer is crucial because it provides real-time feedback on the train's speed and deceleration, which helps engineers monitor the effectiveness of the dynamic braking system. Without this feedback, it can be challenging to assess how much braking force is being applied and how quickly the train is decelerating. This information is vital for maintaining control, ensuring safety, and making necessary adjustments to the braking strategy as conditions change during the train's operation.

While having a trained crew member in dynamic braking, additional air supply, or a secondary emergency brake system may be beneficial in certain circumstances, they do not directly enhance the operational effectiveness of dynamic brakes in the same way that an accelerometer does. Therefore, the accelerometer is essential equipment that directly supports the successful application of dynamic braking by providing the necessary data for informed decision-making during train movements.