The loss of third rail power on a subway car first affects which component?

The correct answer is directly related to the function and dependency of the subway car's systems on third rail power. The air compressor is responsible for generating compressed air, which is crucial for the operation of brakes and other systems. When third rail power is lost, the first noticeable impact is on this compressor, as it is typically reliant on electrical power to function.

In many systems, the air compressor is one of the primary components that ceases operation when there is a loss of power, since it does not usually have a reserve supply of energy or an alternative power source available to it immediately upon power loss. This means that functions dependent on compressed air, like braking power, could be impacted quite rapidly after losing third rail power.

While auxiliary power supplies, train control systems, and emergency lighting may also experience issues following a loss of power, they often have mechanisms in place to either maintain a minimal level of operation or to switch to backup systems. The air compressor, however, typically fails immediately when third rail power is unavailable, making it the first component affected by such an event.