What does Rule 101 permission indicate?

Rule 101 permission is indicative of a specific signal situation where the train or engineer is permitted to proceed with caution. In this case, the correct choice refers to a signal displaying a double red over solid lunar white light, which typically indicates that the train must approach a restricted section of track with an awareness of potential hazards ahead.

This signaling configuration operates within a framework designed to ensure safety while allowing movement under controlled conditions. It serves as a warning that the engineer must maintain control and be prepared to stop as required due to potential conflicts or restricted movements on the track.

In contrast, other signals mentioned indicate different operational statuses. For instance, a single red light typically signifies a stop, a green light indicates that the train may proceed at a normal speed, and a flashing yellow light usually serves as an alert for the engineer to prepare to stop for a red signal. Understanding the significance of Rule 101 permission and the corresponding signal assists train engineers in making informed and safe decisions while operating the train.