In message , Sargeant Rutter writes

Its a "calling on light" it illuminates to tell the driver of the train
behind, that the train is in trouble of some kind, and will call the
train behind forward to assist. Once that light is on, the assisting
driver is assured that the train will not move.

No, that's the orange light above the window.

I did know what the other light was for (as it is a light) but as we
never use it, I've forgotten, like chunky monkey.
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)