What regulates the maximum prescribed pressure when charging the train brake system?

The maximum prescribed pressure when charging the train brake system is regulated by the regulating valve. This component is crucial because it ensures that the air pressure delivered to the brake system does not exceed a set limit, which is vital for the safe operation of the train.

The regulating valve plays a key role in controlling and maintaining the appropriate air pressure as the system is charged. If the pressure were to rise above the recommended levels, it could lead to failure of the air brake system or even pose risks to safety during train operation. Therefore, this component is engineered to effectively limit the maximum pressure, ensuring that the system operates within safe parameters.

Other components like the control valve, safety valve, and pressure gauge serve different functions. The control valve directs the flow of air in the braking system, the safety valve provides a fail-safe by venting excess pressure, and the pressure gauge simply displays the current pressure levels but does not regulate them. Thus, the regulating valve is specifically designed to manage the maximum allowable pressure, making it the correct answer.