Final answer:
The train operator should stop and remain stationary upon seeing a white light moved rapidly up and down next to a STOP signal in the subway to ensure safety.
Step-by-step explanation:
On approaching a home signal indicating STOP in the subway, if the train operator observes a white light being moved rapidly up and down alongside the signal, the train operator should become stationary and stop until it is safe to proceed.
This signal indicates that there is something requiring the train to halt, ensuring the safety of the passengers and the train itself. It is imperative that operators adhere to signal instructions to prevent potential accidents on the tracks.