Final answer:
The most likely outcome of a third rail power failure is that the subway will stop running due to its reliance on continuous electrical power for propulsion.
Step-by-step explanation:
If the power supply for the third rail in the subway fails, the most likely outcome is that the subway will stop running. This is because subways typically depend on a continuous supply of electricity from the third rail for propulsion, lighting, and other critical systems. While some trains have backup batteries, they are usually for emergency lighting and systems, not for the train's propulsion. The options of the subway continuing to run on battery power, switching to an alternate power source, or running at a reduced speed are less likely to be viable, as subways do not generally have these capabilities built into their standard operating procedures or infrastructure.