What does NOT qualify as readily visible for blue signal compliance?

The blue signal compliance requirements specify that signals must be clearly visible to ensure safety during maintenance or servicing of railroad equipment. Signals must be positioned in a way that they can be easily seen and interpreted by anyone approaching the work area, especially by those who may be operating the train or vehicle.

In this context, a blue flag attached to the Engineer's window does not qualify as readily visible because it is typically positioned within the confines of the cab of the locomotive. Therefore, it is not visible to personnel working on or near the tracks. For effective compliance with blue signal rules, the signals must be located where those who need to see them can do so without obstruction, allowing them to understand that work is being performed and that they should not move the equipment.

In contrast, the other options provided facilitate visibility, either by being displayed at the extreme end of the equipment or in advance of it, or being attached to the controlling engine, all of which ensure that the signal is observable from a distance and alerts individuals of the maintenance status effectively.