A train operator can key-by which type of signal?

The correct answer is automatic signal. Automatic signals are essential for train operations as they provide real-time information to the train operator regarding track conditions, speed limits, and the status of upcoming signals without requiring manual intervention. The train's onboard systems can detect these signals and respond accordingly, ensuring safe and efficient train movement.

Keying-by automatic signals allows operators to proceed when they are properly aligned with the conditions indicated by the signal, enhancing safety and operational flow on the rail network. Since automatic signals do not require the train operator to manually acknowledge or set them, they streamline operations and reduce the chances of human error.

In contrast, while manual signals do require operator intervention and may involve some form of acknowledgment, they are not typically "keyed by" in the same way that automatic signals are, since they convey information that a train operator must interpret and act upon themselves. Warning and emergency signals serve specific purposes, providing alerts or instructions during critical situations, but they are not the types of signals typically keyed by operators in normal operational contexts.