What color must the exterior ATC indicator light lens be?

The correct response regarding the color of the exterior Automatic Train Control (ATC) indicator light lens is blue. This color is specifically designated to provide clarity and uniformity in rail operations, allowing engineers and maintenance personnel to easily identify the status of the ATC system. Blue signals are commonly associated with the operation of the ATC, enhancing safety and communication between the train control mechanisms and operators.

Other color designations, such as red, green, and yellow, serve different functions and indicate varying states of operation or warnings within the railway system. However, the blue lens remains crucial for the distinct identification of the ATC indicator feature, reinforcing the importance of using standardized color coding in railway safety systems.