What action does the stop signal followed by "key-by" require from the train operator?

The action required by the train operator when encountering a stop signal followed by "key-by" is to stop and then continue past the signal. This instruction indicates to the operator that, while the signal indicates a stop condition, there is a protocol allowing for the train to pass the signal after halting.

Typically, this scenario occurs when there is a need for the train to proceed despite the stop indication, often due to operational requirements or after confirming that it is safe to continue. The operator must first bring the train to a complete stop, observe any additional safety requirements (such as checking for any approaching dangers), and then proceed past the stop signal at a controlled speed, often referred to as "keying by" the signal.

In this context, stopping ensures safety is prioritized, while continuing afterward allows for flexibility in the operation of the train within established protocols. The other options suggest either prolonged halting, misunderstanding of the signal's intent, or inappropriate speed adjustments, which do not align with the proper procedure for a stop signal with the "key-by" instruction.