What indicates that third rail power has been removed on PA4 cars?

The correct answer emphasizes the indicators that show third rail power has been removed from PA4 cars. When third rail power is disengaged, the systems that rely on that power supply will no longer function. This includes essential components such as the lights, air compressor, and motor power. Their absence suggests that the car is not receiving the necessary power from the third rail, confirming the loss of that power source.

In contrast, the other options include scenarios where lights or motor power are still present, which would not be indicative of third rail power being removed. Flashing emergency signals may suggest a different alert condition that does not directly relate to power status. Active systems normally dependent on third rail power would definitely contradict the situation where that power is absent. Therefore, the absence of these critical systems correctly signals that third rail power has indeed been removed.