Under which circumstances can a train pass a signal?

The correct choice reveals circumstances under which a train can pass a signal, particularly focusing on specific signal numbers. In railway operations, certain signal numbers are designated as "permissive signals," which allow a train to proceed under specific conditions even if they would typically be expected to stop. These signal numbers, such as 279, 101, and 253, have established meanings and rules associated with them that clarify when and how they can be passed.

Understanding this context is critical because it emphasizes the importance of recognizing various signal indications and the established protocols that govern train movement. The signals reflect the rules of the railway system that ensure safety and efficiency in train operations, highlighting the significance of knowing specific signal numbers and their implications for train operations.

In contrast, other options refer to more general circumstances that do not universally allow a train to pass signals. For example, a green light typically means go, but not all signals that show a green light indicate a clear line ahead. Scheduled inspection might involve temporary stops or checks rather than passing a signal as a matter of course. Lastly, being instructed by the conductor may be necessary in specific situations but does not apply as a blanket rule for all signals, reinforcing the need for accurate understanding of signal specifics rather than relying solely