Through which component do the signal mains receive their power?

The correct choice is the power distribution unit. The signal mains require a reliable and controlled source of electrical power to operate effectively, and a power distribution unit is specifically designed for this purpose. It serves as the central hub from which power is distributed to various signals and equipment throughout the system. This ensures that all components receive the necessary voltage and current to function properly.

The transfer switch, while important in switching power sources, does not directly provide power to the signal mains. The reset button is a control mechanism that allows users to reset devices but does not supply power itself. The main circuit breaker is crucial for protecting circuits from overloads but is not directly responsible for wherein the signal mains get their operational power. Understanding the role of the power distribution unit helps clarify its critical function in the power supply chain for transit signaling systems.