What is one of the three local indicating lights above the engineer's cab door on M-3 cars?

The top green light above the engineer's cab door on M-3 cars serves a specific and important function in the operation of the train. This light indicates that the train is ready to proceed or move forward, showing that the necessary safety checks and conditions are met for departure. The green light is a universal signal in rail systems indicating safety and permission to operate, ensuring that both the engineer and other personnel are aware of the train’s operational status. This clarity is critical for both operational efficiency and overall safety on the rail network. Understanding the function of this light helps train crew members recognize and adhere to safety protocols during operations.