What needle color indicates brake cylinder pressure on the duplex air pressure gauge?

Answer: B — Red

The needle color that indicates brake cylinder pressure on the duplex air pressure gauge is red. This is a standardized color coding used in many air brake systems, including those found in train operations, to ensure quick identification and avoid confusion during inspections or troubleshooting. When a technician or operator checks the gauge, the red needle clearly shows the pressure in the brake cylinder, which is critical for understanding whether the braking system is functioning properly. Knowing this allows for faster assessments and responses, especially in situations where timely braking capacity is essential for safety. Therefore, recognizing the red needle is crucial for accurate monitoring of the braking system's performance.