When is the whistle allowed to be sounded as a warning?

The whistle can be sounded as a warning whenever necessary, regardless of any prohibitions, as long as it is done for safety reasons. This practice is important in rail operations because the whistle serves as an alert to pedestrians, motorists, and other rail traffic of an approaching train, especially in situations where safety may be compromised.

Whistle bans may exist in specific locations, such as residential areas or near certain crossings, but in emergency or critical scenarios where there's a potential danger, sounding the whistle is permitted to ensure the safety of all involved. This reflects the overarching priority of safety in railway operations, as any confusion or risk to people nearby can warrant the use of the whistle.

The other options suggest limitations or specific conditions under which the whistle can be used, which may not align with essential safety practices where immediate warnings are needed.