A door fault light on a train illuminates when what condition occurs?

The illumination of a door fault light on a train signifies that there is an issue specifically related to the status of the passenger doors. In this case, when a passenger door is not properly closed and locked, it triggers the door fault light as a safety measure. This alert ensures that train operators and staff are notified of the potential risk, preventing the train from departing until the issue is resolved and all doors are secure.

It is essential for the safe operation of the train that all passenger doors function correctly and are properly secured before departure. The other options do not directly pertain to the specific situation that activates the door fault light, as they either suggest general mechanical issues, actions by passengers that may not necessarily result in a fault indication, or imply that everything is functioning correctly, which contradicts the purpose of the fault light itself.