What is the approximate normal reading of the black hand on the duplex air gauge?

The normal reading of the black hand on a duplex air gauge is approximately 70 pounds. This gauge is used to measure the air pressure in the train's braking system, which is critical for safe operation. The 70-pound mark indicates that the system is under sufficient pressure to enable effective braking.

Understanding the operation of the braking system and maintaining the appropriate pressure levels is essential for the safety of train operations. The other values listed, while they could represent different systems or conditions, do not reflect the standard operational pressure required for the safety and effectiveness of the train's braking system.