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

When the third rail fails, it impacts the power supply that the train relies on to operate its electric systems, including air compressors. The third rail supplies electricity to the train, which is essential for starting and running the compressor that provides air pressure for various train functions, including braking systems and passenger services. Without power from the third rail, the air compressors will not start, leading to the inability to generate the necessary pneumatic pressure.

This scenario highlights the importance of the third rail as a critical component of electric train systems. In contrast, the other options do not accurately reflect the most immediate or direct consequence of a third rail failure. For instance, while a loss of power might affect train lighting or brake systems, the direct failure of air compressors is a key operational issue that arises when power is not supplied.