What is the primary function of the signal main in a transit system?

The primary function of the signal main in a transit system is to control signals. This system is essential for managing the flow of train movements, ensuring that trains operate safely and efficiently on the tracks. The signal main provides critical information that dictates when trains can proceed, stop, or slow down, thus preventing collisions and maintaining schedules.

By controlling signals, the signal main effectively coordinates the entire transit network, allowing for real-time adjustments to train movements based on current conditions and demands. This operational control is vital for both safety and efficiency, making option B the correct answer.

Other choices, while related to transit operations, either do not directly pertain to the signal main's function or are secondary concerns compared to the primary role of controlling signals. Power installation, communication with conductors, and relaying information to passengers are important components of a transit system but do not define the main function of the signal main itself.