What is the first function to be affected by loss of third rail power on a subway car?

The loss of third rail power primarily impacts the air compressor first. The air compressor is essential for generating compressed air that powers various systems within the train, such as the brakes and doors. When the third rail power is unavailable, the air compressor cannot operate, leading to an immediate loss of functionality in these critical systems.

While the lighting system and air conditioning do require electrical power, they generally rely on backup systems or batteries that can prolong their operation for a short period despite the main power loss. The traction motors, which drive the train, also depend heavily on third rail power; however, if the compressor fails first, it can lead to issues in braking and emergency protocols that require compressed air. Thus, the air compressor is the first system to show significant impact due to the loss of third rail power.