Today I am going to bore you all with an aspect of my day job.
De-icing units. Yay!
Somebody asked the question on reddit yesterday. I felt obliged to answer. Mr IanVisits got it slightly wrong, but hey, the pundits can't get it right all the time. My enthusiasm/geekery for all things London Underground was part of the reason I joined in the first place; that's part of the joy of being able to take pride in one's train driving career when you've turned something you liked the look of as a kid into The Job.
I won't go into all the technical details and list all the de-icing units on the combine, but certainly where the shiny S stock trains are concerned, the lights that show that the de-icer is on is a little more prominent that on stocks in the past. This is because LED housing for the outside door indicator light is a lot larger than on older stocks, but blue is the colour of choice when it comes to de-icing units on other lines too.
The light can only be observed when the doors are closed and locked by the driver (so you won't see it when the doors have auto-closed to keep the cold out). Also; the light is only illuminated on the car that has de-icing equipment on, so not all trains (or other normal cars) will have this light.
On S stock trains, a particular train with a de-icing car (M2D car, as we technically call it) is marked with a blue spot.
And when are you likely to see these blue lights?
The use of de-icing equipment (and sometimes, in the middle of the night, when sleet trains are put into operation) is a preventative measure we take to make sure that the conductor rails don't get all iced up, as this has the potential to stall trains as the layer of snow/ice would prevent the train's traction equipment from drawing current from the conductor rails. The risk of this happening increases in open sections of the Underground as the temperatures drop into the evening, and into the early mornings. We use information from the Met Office to calculate these risks and decide when the equipment should be switched on. When switched on, de-icing equipment lays a special fluid onto the conductor rails to help mitigate the risk.
"D" boards, found on most open stations, act as reminders for drivers who are driving trains with de-icing equipment. As shown above, "D ON" would indicate that de-icing equipment should be switched on. Thus, "D OFF" (which basically looks the same, just with an "OFF" and all the colours inverted) would indicated the opposite.