London Underground signals with route indicators

Isn't the real question why the top two are separated from the bottom one?

Also, what is the exact purpose of the shrouds?

I can think of three reasons, but none of them seem particularly likely. Since there is no shrouding to the right, shielding the lights from drivers on adjacent tracks (as the shrouding on the red and green) does not seem that likely.

By 'shrouding', I meant the 'collars' (I don't know what the technical term for them is), that are fitted to signals, road and rail. They are open at the bottom so that snow cannot build up on them.

I don't know where the pictured location is but when I worked at Rayners Lane 1973 - 2002 both platform SB starters had them fitted. I asked the question then and no one knew.

Yes, they all light up together. However, if there were to be two diverging routes, the lowest lamp will need to light for both of them (known as the pivot lamp) and then you can't join the hood to the other two.

Can you say something about 'harbour lights' as they apply to railways. I found them mentioned here but it wasn't very helpful.

Surely something that was literally a miniature harbour light would be too easy to mistake for a normal signal (at some distances).

It's as much down to the age of the signal head and when the mechanical design was produced. The arrangement in this thread is the standard Westinghouse design for a three-lamp junction indicator, there were other designs for ones manufactured by LT and provided from other suppliers such as Howells or Metropolitan-Vickers/AEI/GRS.

Not strictly true it requires 3 filaments to light in a outside type signal like the one in the original post. On a tunnel type it requires at least 2 lamps to make the signal clear as they are only single filament lamps.

On a outside type signal junction indicator each lamp has 2 filaments.

If you see the junction lights on with a red aspect yes the signal is failing but only requires a quick relamp of the defective lamps.

They are known as lunar white as the lens is actually blue in colour

LED’s do work in them but a modification needs to be made to the circuit but the powers that be didnt like the idea so we are still using lamps in junction indicators.

Outside lamps are 33.5W and tunnel lamps are 5W

LED’s that work are 3W behaving like a tunnel lamp

The LED’s LU was using were designed to fail short circuit

Also the pivot light at the bottom has a seperate casing for the lamps then the other two

Signals on the Great Eastern Main Line out of Liverpool Street station used to be like that - red one up from the bottom. Somewhere I have a photo showing this.

I do not have a photo of a LU combined stop and distant signal, these have 4 aspects with the stop signal using the top two aspects and the distant signal using the lower two aspects. I recall seeing one (maybe even several) of these just outside of Leyton station, many years ago - as eastbound trains exit the tunnel they encounter a sharp right-hand curve before arriving at Leyton station and in the days before the line was resignalled for automation I used to be able to see these signals whilst enjoying a driver's eye view of the route ahead.

Usually I would see two green lamps (ie: both home and distant), but sometimes the home was green and the distant was yellow / amber, and occasionally solely the home signal lamp was illuminated - and red.

The combined signals here actually comprised two twin-aspect signal heads slightly staggered in a way which I suppose was designed to ensure that a build-up of snow would not block the view of the red.

Although I believe that only the NAME is different, the principle being the same.