Final answer:
At least one contact shoe is 'alive' when touching the third rail, but the exact number is unspecified. The minimum resistance of a safe path when a person comes into contact with 120-V AC must be high enough to limit current below the harm threshold.
Step-by-step explanation:
When a train operator notices that one contact shoe of a car is touching the third rail, they should be aware that at least one contact shoe is 'alive' or energized on that car. However, the exact number of contact shoes that are 'alive' cannot be determined from this information alone, as multiple shoes may be in contact with the third rail to distribute power evenly across them, ensuring redundancy and continuity of power supply.
In a similar scenario for electrical safety, if a man comes into contact with 120-V AC, and does not feel it while wearing rubber-soled shoes, it indicates that his shoes provided enough insulation to prevent the current from passing through his body. The minimum resistance of the path the current follows through the person would have to be high enough to limit the current below the threshold that can be felt or cause harm, typically at least several tens of thousands of ohms.