How are subway car batteries kept charged?

The correct answer highlights the method of charging subway car batteries through the connection of batteries in series with air compressor motors. This process allows the energy generated by the operation of air compressor motors, which are essential for various functions within the subway cars, to be used effectively for charging the batteries. When the motors operate, they generate electrical energy that can be utilized to keep the batteries charged, ensuring that the essential systems in the subway cars, such as doors and emergency functions, have a reliable power source.

This method is efficient because it integrates the battery charging process into the operational needs of the subway system. The batteries can be charged while the train is in motion, minimizing downtime and ensuring that the power reserves are maintained without requiring additional infrastructure or external power sources.

Other methods like using solar panels or manual charging at each station may not provide a consistent and reliable means of charging as they depend on external factors like sunlight availability or operational schedules. Relying on trackside power sources could be feasible but usually involves a more complex infrastructure and may not directly integrate with the subway car systems as effectively.