It means that the train in the platform is signalled to depart the station, information provided for station staff as the driver receives this in the cab.
On sub surface stock the white lights are on the train with the door indicator lights.
The Central Line uses a different system, the white aspect displayed on signal heads advises that the next section is occupied but movement into that section is possible in ATO. Sequence would be red, white, green.
I can only find a photo of a signal post showing a danger aspect, without white, which is unhelpful to commuter
Last Edit: Jan 28, 2021 10:54:23 GMT by Dstock7080