What is one immediate effect on a train when third rail power fails?

When third rail power fails, one immediate effect is that the ventilating fans cannot operate. In electric trains, the power supplied through the third rail is crucial not only for propulsion but also for operating essential systems, including ventilating fans. These systems are vital for maintaining air quality and comfort within the train's interior. When power is disrupted, fans that rely on electrical energy can no longer function, leading to potential overheating and discomfort for passengers, especially in enclosed spaces.

Understanding this impact highlights the importance of continuous power supply for overall train operation and passenger experience. While other systems, like braking or signaling, may also be affected indirectly by third rail failures, the immediate and direct consequence of losing power is the failure of systems dependent on that power, such as the ventilating fans.