If a train sees three yellow lanterns arranged vertically on the right-hand side of the track, what should the operator do?

When a train operator sees three yellow lanterns arranged vertically on the right-hand side of the track, this signal typically indicates that the train should proceed cautiously. The presence of three vertically arranged yellow lights serves as a warning that there may be some condition ahead that requires the train to approach with care, usually implying that there is a flagman or some other form of temporary track maintenance activity in progress.

Therefore, the correct response is to proceed cautiously to the next flagman, allowing the operator to maintain a safe speed and be prepared for any instructions or changes in the track situation that may arise as they approach the flagman. It's important to note that while bringing the train under control and blowing a warning, or stopping, might be necessary in other scenarios, the specific context of the three yellow lanterns directs the operator to maintain a careful approach rather than a complete halt.