Loss of third rail power on a subway car primarily affects systems needing constant electricity, with lights likely failing first. Brakes have fail-safe mechanisms and can function temporarily without power.
Loss of third rail power on a subway car will first prevent functioning of the lights. The third rail is a method of providing electricity to electrified transportation systems such as subway trains and light rail. When this power is lost, the systems that require immediate electrical power such as lighting will fail to function first since they depend on a continuous supply of electricity.
In contrast, some other systems like brakes are typically designed to be fail-safe and can operate, at least temporarily, through other means such as backup batteries or through mechanical/manual activation for safety reasons.
In conclusion, while the loss of third rail power can affect various systems, non-critical features like lights will likely be the first to stop working due to their constant need for electrical power, whereas critical safety systems like brakes are designed to be more resilient to power outages.