I was at Stockwell Northern Northbound yesterday (as the vic was part-suspended).
On the headwall there is a bright white light that seemed to come on as a train drew to a halt and went out when the last car passed it. So far, so signal-like, but it is right out of the driver's eyeline, being over the pedestrian egress tunnel (runs out parallel to the running tunnel), and with a mesh cage of misc equipment between the two tunnels.
Can anybody say what it is? trip-cock tester? Kennington tea-point indicator?
Ahh, I didn't know such a thing existed. I've not noticed one at any other stations - are they widespread, or is Stockwell particularly favoured? (which would make sense as it will be affected by any Kennington carryings-on.
Officially they are referred to as Ready To Depart Indicators (RTDI’s) which should light up approximately 10 seconds before departure coinciding with the "ATO Start Required" message displayed to the driver at the given time.
Most stations (those which have them) on the Jubilee and Northern have them midway down the platform however at some underground stations, RTDI’s are at the headwall for sighting purposes.
With RTDI's? Almost all of the below ground stations on those two lines have them. I say most because I can't recall weather all below ground stations are required to have them but all the stations that usually feature platform dispatch (aswell as termini stations) need to have them for obvious reasons.
If you were asking about RTDI's at the helm of platforms then the Northern line has several with Edgware, Camden Town (SB), Warren St, Goodge Street, Charing X, Waterloo (Northern) and Kennington (NB) to name a few. Clapham North and Clapham Common are a bit more unique in that both RTDI's are at the Southern end of the platforms by the staircase meaning that their Northbound platforms have an RTDI at the rearmost of the platform.