LED Colour-light Signals

[Pyewipe Junction]

Watching cab-view videos on YouTube, I'm noticing the spread of single-aspect LED colour-light signals. How do these signals display 'double-yellow' status? Or don't they?

[Mickey]

Single aspect LED colour light signals won't show a 'double-yellow' aspect because they can't. We've had a couple where I am since late 2009 that replaced a couple of x3 aspect colour light signal heads (the remaining few others are still x3 aspect colour lights). I suppose they are a good idea in so far as if a individual or a few LED filaments (bulbs) go out in the LED colour light signal head the vast majority of the LEDs will still show illuminated to an approaching train driver where as if a individual filament or aspect goes out in a 'ordinary' colour light signal not only does it cause issues at the signal it's self with getting trains to maybe pass a 'blanked out' aspect but it can put the colour light signal in the rear of it back to RED if the 'blanked out' aspect is displayed.

[WTTReprinter]

In 4 aspect areas, an auxiliary yellow is shown using a separate LED unit, in much the same way as the old 'searchlight' signals do.

[StevieG]

Just a cautionary note Pyewipe Junction, WTTReprinter, about use of the term 'auxiliary'.

In case LMS or BR-LMR signal plans are ever encountered which refer to Auxiliary yellows or show a conventional colour-light signal with two yellow lights where beside one of which, "AUX" can be seen, (commonly a yellow/green Distant signal looking on site like a 3-aspect signal), an "Auxiliary Yellow" meant a yellow never illuminated when all was well, but was present as an automatic standby which would illuminate if, when the signal needed to show yellow, the usual yellow lamp/bulb had failed (presumably when only single filament bulbs were used).

But as WTTReprinter has indicated, provision of a second 'lens', which is either not illuminated or showing yellow, on modern LED signals gives them the capability of showing the double yellow indication as a normal function of the signal.

[Mickey]

After reading the above posts it has occurred to me in my area that above the single aspect LED aspect there is enough space to possibly fit another LED aspect thus allowing for a 'double-aspect' indication maybe in the future if the traffic requirements justify it?.

[Pyewipe Junction]

Could someone please explain the rules that govern the circumstances where double-yellow aspect signals are required? I'd hazard a guess that the determining factors would be traffic density and/or line speed.

[Mickey]

Could someone please explain the rules that govern the circumstances where double-yellow aspect signals are required? I'd hazard a guess that the determining factors would be traffic density and/or line speed.

I don't know of any official criteria that determines where 'double-yellow' aspects are required but in British railway signalling and as always there are exceptions usually 'double-yellow' aspects are to be found on 'fast lines' or on 'fast double-track main lines' on Track Circuit Block operated lines. When Absolute Block was still common on the ECML until the mid 1970s for example with lineside signal boxes at irregular intervals along the route usually if two signal boxes were relatively close to each other the fast line colour light distant signal for the first signal box would show a single yellow aspect and a 'double-yellow' aspect for the second signal box (after the first signal box had cleared his home signals) with that same colour light distant signal acting as a 'outer colour light distant signal' for the second signal box. A green aspect would only being displayed in that distant signal when both boxes had pulled off there signals.

As with everything on the railways the above post may not always be the case for example at Hatfield the Up & Down fast lines x2 aspect colour light distant signals would only show a green aspect (no double-yellow aspect) when on the Up fast line both Hatfield No.1 & No.2 had both 'pulled off' along the Up fast line then the Hatfield colour light distant signal south of the 'Sand hole bridge' would change from a single yellow aspect to a green aspect likewise on the Down fast line the Hatfield colour light distant signal back at Redhall would only show a green aspect (no double-yellow aspect) when both Hatfield No.1 & No.3 had both 'pulled off' along the Down fast line.

[WTTReprinter]

It's a design issue when resignalling takes place.
They look at planned speeds and service density to see how far to space signals apart. Then they look at the 'headway' (the point between the stop signal and the yellow) and if it isn't enough to brake to a stop they increase the distance by putting YY further back. In fact, in really dense areas such as the GE where I started my career, there were places where a driver might see two or even three YY on approach to a red.
There are many more considerations in planning a signalling scheme but that's a brief outline.

[Mickey]

Yes indeed WTTReprinter you are more succinct in your post as opposed to mine where I was thinking more of the ECML during the 1960s & 1970s.

[StevieG]

"It's a design issue when resignalling takes place.
They look at planned speeds and service density to see how far to space signals apart. .... "

Quite so, that's why Mickey, as WTTReprinter hints at below, YYs can be needed on relatively slow speed lines with dense traffic, which have therefore been equipped with signals separated by much reduced distances.

" .... In fact, in really dense areas such as the GE where I started my career, there were places where a driver might see two or even three YY on approach to a red. .... "

The need for, and number of, YYs that might be needed leading to a red, sometimes had to be increased if dense traffic lines with a frequent but often stopping passenger service, also had to cater for non-stopping trains of a type that needed longer distances in which to stop safely.

When the GE Main line in London and Essex was resignalled in the 1990s, between Stratford and Gidea Park, this sort of traffic mix was catered for on the Electric Lines (similarly to that of the late 1940s signalling, but slightly enhanced) by a signal which was being maintained at red being preceded by the following sequence : -
R** - R*** - Y - (up to 4) YYs.

** - the signal being maintained at R
*** - but ready to clear to Y by approach release

[Mickey]

What about the 'flashing double-yellow aspect' they wasn't introduced on fast lines until the late 1970s or 1980s I believe?. I don't really remember seeing them much and I presume they were the colour light equivalent to a 'splitting distant signal' for a fast line 'turn in' at an advance junction.

[StevieG]

What about the 'flashing double-yellow aspect' they wasn't introduced on fast lines until the late 1970s or 1980s I believe?. I don't really remember seeing them much and I presume they were the colour light equivalent to a 'splitting distant signal' for a fast line 'turn in' at an advance junction.

Not really what Pyewipe Junction asked about, but yes Mickey, I think it was, at least in part, the HSTs that drove the need for some different solution for 'high' speed junctions, which ended up being flashing yellows.
On the GN in Hertfordshire, the 70mph 'turn-ins' at Digswell, Woolmer Green, and Hitchin South (Down) soon got flashing yellows [ previously, the approach aspect to the 'junction' signal (which was held at Y until train approached of course), rather than the later FL-YY leading to FL-Y aspects before the 'junction' signal, there had just been one YY].

[thesignalman]

Could someone please explain the rules that govern the circumstances where double-yellow aspect signals are required? I'd hazard a guess that the determining factors would be traffic density and/or line speed.

In two words: Braking Distance.

If a train would not be able to stop at a red signal after passing a yellow, then a double yellow is placed in the next signal back. There are a few locations where you pass two double-yellows before reaching a yellow to ensure adequate braking distance.

John