The black hand on the duplex air gauge indicates the pressure in which system?

The black hand on the duplex air gauge specifically indicates the pressure within the brake pipe system. This is crucial for train operation since the brake pipe pressure is an essential component for controlling the braking system. As the pressure in the brake pipe decreases, it signals to the train's braking system that a reduction in brake force is necessary, allowing for effective management of the train's speed and stopping distance.

In the context of train operation, understanding the pressure readings in the brake pipe is vital for ensuring safety and proper functionality. The data from this gauge helps the train operator monitor the system's operation and respond appropriately to any changes.

Other systems listed in the options, while also important for overall train operation, do not use the black hand on the duplex gauge to display their pressure readings. The dynamic brake, air conditioning, and emergency brake involve different mechanisms and pressure readings within distinct systems.