What indicates that third rail power is removed on PA4 cars?

The correct indication that third rail power is removed on PA4 cars is signified by the absence of lights, air compressor, and motor power. This is a critical safety feature designed to ensure that train engineers can quickly recognize when power is not available, allowing them to take appropriate actions.

When third rail power is removed, the systems reliant on electrical power—such as lights and the air compressor, which controls critical functions like the brakes—will not operate. Additionally, the train's motors would be unable to function, effectively rendering the train inoperable. This absence of power and functionality is vital for safety protocols on the train and helps prevent potential accidents or mishaps due to unawareness of power status.

The other options do not correctly reflect how train engineers determine the status of third rail power: lights and air compressor presence suggest that power is still active, audible alerts alone may not provide a comprehensive assessment of the power status, and flashing indicators might not convey the complete situation without the context of operational functionality.