The black hand of the duplex air gauge on a subway car indicates what?

The black hand of the duplex air gauge on a subway car is crucial for understanding the air pressure in the brake pipe. This gauge provides vital information to the operator about the current pressure status of the brake system, which is key for ensuring that the brakes function properly and respond effectively when applied.

Monitoring the air pressure in the brake pipe is essential for safe train operation, as it affects the train's braking capabilities. If the pressure is too low, braking efficiency can be compromised, leading to potential safety hazards. Thus, the indication provided by the black hand allows operators to assess whether the brake system is in proper working condition, ensuring safety for both the passengers and the train itself.