How common are flashing yellows?

[phil-b259]

Thanks for the replies. Now were those previously mentioned flashing yellows always there, or were they introduced when the signal heads were updated to LEDs recently?

 

Dave

 

Flashing Yellows are not determined by the signal heads - the functionality (be it extra coding in the software in SSI / CBI interlockings or the extra relays + wiring in older schemes)  must be installed during the installation of the wider signalling system. In other words the interlocking / lineside location cases must have the provision for it designed in at the outset - and modifying or renewing these to retro fit such functionality doesn't come cheap, certainly far to great to simply go round sticking in flashing yellows on a whim. As such if a site has new LED signals that have flashing yellows, the previous signals will also have used flashing yellows unless the LED signals are being installed as part of a wider resignalling scheme where wholesale changes are being made to interlockings etc..

 

People should note that most LED railway signals (and indeed rad traffic signals) are designed to be a like for like replacement of previously installed filament lamp based versions with the cabling connecting it to the lineside location case being identical and with no change being made to the voltages etc supplied to the LED signal precisely to avoid having to do any 'surgery' to the signalling system itself (and all the extra costs that entails).

[Edwin_m]

So the initial answer was to introduce what was known as a free yellow at the junction signal by allowing it to clear when the route was set but holding the signal in advance of the junction at red (if it wasn't already red for some other reason) thus a train would in theory be braking down to the divergence speed in any case in order to stop at the next signal in advance.  But this too wasn't entirely satisfactory and certainly didn't work for much higher divergence speeds such as 70 mph turnouts.

As I understand it this is where the "better braking of the HST" came in.  The positioning of the free yellows relative to the junction speed were designed around trains following the normal braking curve applicable to stock with nominal 7%g braking.  HSTs with 9%g brakes (and most later stock had them too) could pass the junction signal that much faster and still be confident of stopping at the red beyond, but would have passed the junction too fast in the meantime. Flashing yellows were the solution to this problem, albeit still imperfect. 

 

Incidentally I believe the first Preliminary Routeing Indicator was actually simultaneous with the revival of the splitting distant, both being provided at Stockley for the junction to Heathrow.  I don't recall the exact details but it was something to do with both routes being high speed but if an electric unit was routed off the wires it would be embarrassing and if a diesel was routed into the tunnels it would probably be worse than that. 

[Zomboid]

What are splitting distants? What do they look like and how do they work?

[BobM]

Hi Guys...Slightly off topic but with the 'debate' over whether it's yellow or amber........in mind....

 

This reminded me of when I used to design road traffic and motorway signs and made up contract lists, we used to have a (slightly) amusing but none PC term for the yellow school flashing warning units....

 

Twin Amber Flashing Light Units (TAFLU's)......the 'technical term' being always  'Welsh Toilets' ----

[TheSignalEngineer]

This is a splitting distant under a stop signal in semaphore signalling.

911430 60009 at Larbert A4_Keith Sanders.jpg by Keith Sanders, on Flickr

 

The distant arm below the Stop arm is for the straight route. In this case the splitting distant is 'Off' denoting that the points at the next junction are set for the left hand diverging route and the junction signal is 'Off'

 

Similar signals exist in colour light areas, having a second offset head which doesn't have a red aspect.

 

Isolated splitting distants would have just the two yellow arms or two Y/G colour light heads.

[Zomboid]

Thanks.

[The Stationmaster]

As I understand it this is where the "better braking of the HST" came in.  The positioning of the free yellows relative to the junction speed were designed around trains following the normal braking curve applicable to stock with nominal 7%g braking.  HSTs with 9%g brakes (and most later stock had them too) could pass the junction signal that much faster and still be confident of stopping at the red beyond, but would have passed the junction too fast in the meantime. Flashing yellows were the solution to this problem, albeit still imperfect. 

 

Incidentally I believe the first Preliminary Routeing Indicator was actually simultaneous with the revival of the splitting distant, both being provided at Stockley for the junction to Heathrow.  I don't recall the exact details but it was something to do with both routes being high speed but if an electric unit was routed off the wires it would be embarrassing and if a diesel was routed into the tunnels it would probably be worse than that. 

 

I've an idea the PRI was (and is??) some way in rear of the splitting distant.  The main idea was to allow an electric unit to continue to run at maximum speed without having to slow in case the route to the airport wasn't set at the junction as it was considered that the distance between the splitting distant and the junction wasn't sufficient for it to stop in rear of the junction from maximum speed.  This in turn resolved a  timetabling problem caused by the speed differential and acceleration differential between an HST and the HEX units which in order to achieve optimum pathing required the latter to run at their full speed right up to the point where they braked for the divergence at Airport Jcn.

[jim.snowdon]

I think the whole issue originates in the changes that occurred between semaphore signalling and modern colour-light signalling, in that with the former, the route could be indicated at the distant, which was set far enough back from the home signal that the train could be stopped at the latter if the wrong route had been set. Wind the clock forward to modern, ie BR colour light signalling, and the route indication had moved to the last signal before the junction, ie the equivalent of the splitting home signal. If that was approach released, the first indication of the route set was as the driver approached the junction signal. That was alright so long as it was set at red, clearing to an appropriate indication on approach, but it does slow the train down quite considerably, and becomes more and more of a problem as junction speeds increased. The use of the flashing aspects improved the situation by allowing the junction signal to be approach released from a single yellow, whilst at the same time indicating to the driver, by the flashing aspects, that the diverging route was set as far out as the double yellow. The whole arrangement still presumes there to be a fairly significant reduction of speed for the diverging route.

 

The PRI effectively separates the provision of route information from the signalling system, allowing the latter to show the correct aspects according to the occupation of the line ahead and leaving it to the driver to adjust the train speed to keep it within the limit prescribed for the turnout. Interestingly, I spent a bit of time in the Signal Design Office at Reading some years ago, and found that there had been quite serious consideration of an alternative solution that would have put a route indicator on the signal in rear of the actual junction signal, in effect giving the drivers the feathers one signal section earlier than previously.

 

Jim

[The Johnster]

What are splitting distants? What do they look like and how do they work?

 

All power to the Hypnotoad, Zomboid.  See Signal Engineer's reply, and the very good photo of the A4 at Larbert.  A splitting distant would be provided where the diverging, facing 'branch' route could be traversed at 40mph or more; if the facing junction was speed restricted lower in that direction, as the huge majority were, a 'normal' single distant would be provided and set to caution for any train approaching that was going to diverge on to the branch; it would slow so as to be able to stop at the home if necessary, and the signalman would clear the appropriate arm of the splitting home signal as it approached.  In some places the signalman was instructed to bring the approaching train to a stand before clearing the home.  In cases where both routes through the junction were speed restricted lower than 40mph, it would be a fixed distant; the presence of a clearing distant signal indicates a 40mph or higher permitted speed on a route it is cleared for.

 

Drivers' route knowledge of course includes the locations of such signals and the permitted speeds through the junctions to which they apply, in both facing and trailing directions.  No particularly special signalling arrangements are required in the trailing direction, just normal homes and trap points protecting the junction.  The splitting distants mostly looked like splitting homes with yellow distant fishtail boards and one post lower than the other indicating the direction of the junction, left in the case of the Larbert photo, but right for (as an example) the down Badminton route at Wootton Bassett (it wasn't Royal in those days).

 

In model terms, the usefulness of a splitting distant is limited; few of us have the space to model high speed junctions to scale in a way that would include them, as they are at least 440 yards back from the home, and most of us model station or junction areas rather than plain main line.  A possible application might be something like the Larbert example, where the actual high speed junction is assumed to be 'off stage' somewhere in advance of the scenic break, but real life examples of the Larbert setup where the main 'straight ahead' distant is below the starter for a short section are not common.

 

Happy to explain further if you need it, so long as the Hypnotoad alllows it of course...

[LNERGE]

Here's my favourite splitting distant.. It normally displayed one red light. When Ely Station North pulled off it went to three yellows. Here Ely North Junction has cleared the route onto the Norwich line..

 

https://www.flickr.com/photos/nicholasy/9040622428/

[The Stationmaster]

I think the whole issue originates in the changes that occurred between semaphore signalling and modern colour-light signalling, in that with the former, the route could be indicated at the distant, which was set far enough back from the home signal that the train could be stopped at the latter if the wrong route had been set. Wind the clock forward to modern, ie BR colour light signalling, and the route indication had moved to the last signal before the junction, ie the equivalent of the splitting home signal. If that was approach released, the first indication of the route set was as the driver approached the junction signal. That was alright so long as it was set at red, clearing to an appropriate indication on approach, but it does slow the train down quite considerably, and becomes more and more of a problem as junction speeds increased. The use of the flashing aspects improved the situation by allowing the junction signal to be approach released from a single yellow, whilst at the same time indicating to the driver, by the flashing aspects, that the diverging route was set as far out as the double yellow. The whole arrangement still presumes there to be a fairly significant reduction of speed for the diverging route.

 

The PRI effectively separates the provision of route information from the signalling system, allowing the latter to show the correct aspects according to the occupation of the line ahead and leaving it to the driver to adjust the train speed to keep it within the limit prescribed for the turnout. Interestingly, I spent a bit of time in the Signal Design Office at Reading some years ago, and found that there had been quite serious consideration of an alternative solution that would have put a route indicator on the signal in rear of the actual junction signal, in effect giving the drivers the feathers one signal section earlier than previously.

 

Jim

 

We examined a number of alternatives for preliminary route indicators on the Reading Remodelling Working Group in the early 1990s with some very good ideas coming from a particular Signal Engineer although they were increasingly sat on by another engineer - I suspect mainly on the NIH principle.  I think I might still have some of the Signalling Facilities Drawings (SFDs) that were worked up by the Group back then.  One particular problem was the succession of relatively closely spaced running and physical  junctions with varying speeds where we all required trains to be kept moving at optimum speeds whichever route they would be taking one or two signals in advance.

 

But of course only a few of our layout proposals ever appeared in reality due to financial problems and then the present scheme came along with a nice blank sheet of paper and a very large pile of money to pay for it - but we can claim as still surviving Tilehurst East Junction and the splitting distants for it.

 

Mind you even dealing with things once the principles were well established could get quite complex and Continental Junction (towards the Channel tunnel posed some interesting challenges when we were running SPAD review and mitigation meetings in connection with, mainly, CTRL related works in my post 'big railway' days when I was working for a signal engineering company.

[D6975]

The signal controlling the approach to the junction at Patchway (heading towards TM or Parkway) has had a flashing yellow when set for TM direction for donkey's years.  I first noticed it sometime in the late 80s, I don't know how long it had already been there.

[Swindon 123]

Here's my favourite splitting distant.. It normally displayed one red light. When Ely Station North pulled off it went to three yellows. Here Ely North Junction has cleared the route onto the Norwich line..

 

https://www.flickr.com/photos/nicholasy/9040622428/

Here is the all yellow version.

 

And another form of splitting distant on the end of Clapton platform, acting as a splitting distant for Clapton Junction H17 signal. The route is set for the Chingford line.

 BBR-11-119 by Paul James, on Flickr

 

H17 signal at Clapton Junction.

BBR-11-120 by Paul James, on Flickr

 

 

Paul J.

[Edwin_m]

H17 signal at Clapton Junction.

BBR-11-120 by Paul James, on Flickr

 

 

Paul J.

Is that one actually a splitting distant?  Presence of a red and green together suggests to me that it is a junction signal that pre-dates the use of "feathers", similar to the semaphore junction signal with two red arms alongside each other. 

 

Even where modern splitting distants are provided, the junction signal itself still has a single set of lenses with a junction or route indicator.  Allowing a driver to pass a red at speed is considered to be a safety risk. 

[Swindon 123]

Is that one actually a splitting distant?  Presence of a red and green together suggests to me that it is a junction signal that pre-dates the use of "feathers", similar to the semaphore junction signal with two red arms alongside each other. 

 

Even where modern splitting distants are provided, the junction signal itself still has a single set of lenses with a junction or route indicator.  Allowing a driver to pass a red at speed is considered to be a safety risk. 

You are correct Edwin-m, it is a plain old junction signal although technically as they were 3 aspect signals both acted as distants for the next signal on their respective routes, for the LH one it was at Copper Mill Junction.  I just included it to show the sequence from H15 signal previously. Although both colour lights, they mimic semaphore signal indications without the arms, and in the case left hand aspect of H15 also acts as an outer distant for Copper Mill Junction, if you like look at it in terms of semaphore signalling. An early system of colour light signalling installed by the LNER, basically mimicking semaphore signals without the actual signal arms, and the added advantage of 4 aspect indications, allowing for a more intensive service, and also cutting down on the number of signal boxes required.

 

Paul J. 

[beast66606]

BR 29775 - leaflet.

 

Signalling at Junctions : New flashing aspects.

 

1978.

 

 

Appears to be the date for the introduction of the flashing aspects

[The Stationmaster]

BR 29775 - leaflet.

 

Signalling at Junctions : New flashing aspects.

 

1978.

 

 

Appears to be the date for the introduction of the flashing aspects

 

Sounds about right to me Beast - one of my Drivers at my then depot made it some sort of personal mission to keep on moaning that the information in the leaflet was incorrect and kept on about it for months, and months, and months - nothing changed.

 

It is illuminating (or actually not) to remember that for many years splitting distants were outlawed in new colour light installations and were - if I remember rightly - specifically forbidden in the signalling Principles.  The Western had planned to introduce them at Wootton Bassett under the Swindon resignalling scheme and the signal post with fittings and signal heads was actually erected but never commissioned in that form after someone apparently raised the point that the signal was not compliant with then current Principles (notwithstanding the fact that splitting distants existed elsewhere under earlier hemes as illustrated in post above.

 

Instead a splitting banner repeater was dropped into the scheme - 1130yds in rear of the signal it applied to (SN62) but I've an idea that too might have been canned; years later I'm reasonably sure things were solved with a flashing yellow as the junction linespeeds were 90mph/70mph even in 1969.

[EddieB]

Interesting to recall this thread the other day, as I had the opportunity to do a bit of photography at Cheddington and noticed the flashing yellow on the down fast/platform 1 road. From what I could work out on the day is that the flashing yellow aspect was only shown to passing trains due to make a station stop at Leighton Buzzard.

[Chris116]

There are flashing yellows at Woking on platform 4 (and possibly 5) for trains routed towards Guildford and also on platform 1 at Surbiton in the Waterloo direction for trains being routed off the slow line onto the fast line.

[Davexoc]

Interesting to recall this thread the other day, as I had the opportunity to do a bit of photography at Cheddington and noticed the flashing yellow on the down fast/platform 1 road. From what I could work out on the day is that the flashing yellow aspect was only shown to passing trains due to make a station stop at Leighton Buzzard.

That would probably indicate they were being switched to the down slow at Ledburn junction prior to the Leighton Buzzard stop.

Where they 110mph units first stop LB?

 

Dave

[Zomboid]

There are also flashing yellows on the approaches to Pirbright and Worting junctions.

Obviously a popular feature on the SWML. Not sure why they're needed at Worting though, the neither route is particularly low speed (Andover gets the feather & flashing aspect -even though the layout suggests that Southampton is the diverging route). I'm pretty sure that Pirbright gets a flashing double yellow, too.

[beast66606]

BR 29775 - leaflet.

 

Signalling at Junctions : New flashing aspects.

 

1978.

 

 

Appears to be the date for the introduction of the flashing aspects

 

And thanks to eBay I now own a copy of the said BR29775

[EddieB]

That would probably indicate they were being switched to the down slow at Ledburn junction prior to the Leighton Buzzard stop.

Where they 110mph units first stop LB?

 

Dave

Yes, I think they were class 350 units heading for Birmingham, first stop Leighton Buzzard.

[The Stationmaster]

Proper use of a flashing yellow?  One I observed at Ealing Broadway the other evening - for an empty stone/ballast train heading for the turnout off the Up Relief onto the Avoiding Line at Acton West; excellent idea I thought as he was going like the clappers.

[DY444]

There are also flashing yellows on the approaches to Pirbright and Worting junctions.

Obviously a popular feature on the SWML. Not sure why they're needed at Worting though, the neither route is particularly low speed (Andover gets the feather & flashing aspect -even though the layout suggests that Southampton is the diverging route). I'm pretty sure that Pirbright gets a flashing double yellow, too.

 

Worting was I believe equipped when the new Basingstoke panel came into use and is probably to avoid the embarrassment of those odd occasions in the past when an emu was given the Salisbury road.