In the C-3 system, what does red indicate when it is the only light on?

When the red light is the only one illuminated in the C-3 system, it indicates that the doors are open. This is a critical safety feature as it alerts the train crew and passengers that the doors are not secured, which must be addressed before the train can safely depart or operate. The presence of the red light serves as a visual cue to ensure that all safety protocols are being followed, particularly concerning the doors' status.

In the context of train operations, it is essential to confirm that doors are closed to prevent accidents or injuries to passengers. Thus, the activation of the red light signifies that personnel must take action to ensure passenger safety and compliance with operational procedures.