If one shoe of a subway car touches the third rail, how many shoes are alive on that car?

A subway car typically has two sets of shoes that make contact with the third rail, which supplies power to the train. Each car is equipped with a positive shoe and a negative shoe. If one shoe touches the third rail, it means that the power supply is engaged through that contact.

In this scenario, the car has shoes designed for both positive and negative connections. Thus, if one shoe makes contact, it indicates that the system is alive, and both shoes are functioning to complete the electrical circuit. Consequently, while there are two active shoes mentioned, when accounting for the entire functionality and design of the system and possibly other systems on the car, one might consider there are four shoes alive, as there are two pairs in typical electric systems.

Understanding that the presence of both a positive and negative shoe functioning makes the car "alive" is critical in this context. Therefore, recognizing that the answer reflects the operational and electrical aspects—specifically the shoes' roles in completing the circuit—explains why the provided answer aligns with how subway electric systems are designed.