A non-illuminated sign can signify the start of which procedure for trains?

The correct understanding is that a non-illuminated sign typically indicates the start of a coasting procedure for trains. Coasting refers to the operation of a train without active propulsion, where the train relies on momentum to continue moving forward. A non-illuminated sign suggests that there are no immediate alerts, hazards, or restrictions, allowing the train operator to reduce power and allow the train to coast.

In the context of train operations, recognizing the meaning of signs, both illuminated and non-illuminated, is crucial for maintaining proper speeds and ensuring safety. Since the sign does not emit light or provide active warnings, it implies a situation where the train can safely transition into a coasting mode without the need for immediate intervention or a change in operational status as required by the other options.