Final answer:
The black hand of the duplex air gauge on a subway car indicates the air pressure in the main reservoir, which is essential for brake operation and door functioning.
Step-by-step explanation:
The question pertains to the duplex air gauge found on subway cars, which is an instrumental aspect of a subway's pneumatic system.
The black hand of the air gauge is an indicator that typically shows the air pressure in the main reservoir. The main reservoir air pressure is critical for several functions, including the operation of the brakes and the opening and closing of doors.
In a duplex gauge, there are generally two hands or pointers.
The black hand indicates the higher pressure reading from the main reservoir, which is essential for the aforementioned functions, while a red or other color hand would indicate brake pipe pressure, which directly controls the brake system.
Proper air pressure ensures the safe and efficient operation of the subway car.
The complete question is:content loaded
The black hand of the duplex air gauge on any type of subway car indicates the air pressure in the