Can 3 lens signals display preliminary caution?

[izvor]

The 'searchlight' signals had a swinging frame between the bulb and the lens containing three colour filters. The frame by weight and springs was normally in a central position showing red. A solenoid moved the frame in either direction to give the yellow or green aspects - when unpowered the frame moved back to the 'red' position.

Interesting, thank you.

 

[Taunton]

The Searchlight was a real bit of 100-year old USA electro-mechanical ingenuity, where I think Union Switch & Signal initially devised and patented it, and the first UK installations, by the LNER, were imported, later being franchised to someone like Westinghouse in Chippenham. The lamp shone through the lens all the time. The colour filters in between were in a U-shaped frame which used a bit of old technology that handled three results. When positive current was in the control wire from the signalbox it swung one way to the green filter. When negative current was applied it swung to the opposite end and displayed the yellow filter. When no current the counterweight swung it back to the central position and showed the red filter. So it was fail-safe as well.

You could see this on the GEML signals that as they stepped up from yellow to green there was a brief flash of red. Unfortunately the first TPWS implementation there used, for whatever reason, a connection directly to the mechanical slide with the filters. It could happen that just as a train approached the signal stepped up from yellow to green, by no means uncommon, and just as it was passing the TPWS ramp it gave a red. Brakes full on. There had to be quite a bit of re-engineering to overcome this.

Because of the current draw by the new colour light signal, powered from lineside batteries, away from busy stretches they often used another bit of cleverness called Approach Lit, where they stayed dark and unlit until a train was in any of the nearby track circuit sections, when it would light up. Supposedly in time for any train to see it, but Gerry Fiennes in his books wrote of one searchlight on the straight March to Spalding line which would come on late, and approaching crews would make bets with one another on what aspect it might finally deign to display! Separately, they were found to be a useful indication to PW crews of a train approaching when they lit up. Certainly in Canada I've seen whole gantries of multi-aspect junction signals all light up a few minutes before a train comes.

 

[py_megapixel]

I may be misremembering but I think at Cheadle Hulme junction and elsewhere as well, there are incandescent colour light signals with two lenses but four aspects

 

[jfollows]

If you mean the attached (https://s0.geograph.org.uk/geophotos/04/52/43/4524388_e02f9eb4_800x800.jpg) they’re not LED 4 aspect signals. A “dichroic mirror signal head” apparently (https://www.networkrailmediacentre....g-system-commissioned-on-west-coast-main-line).

Ansaldo implementation, which failed completely for a number of days a few years ago (https://www.railforums.co.uk/threads/crewe-delays-16-sep.209188/), so not thought to be all that reliable.

Picture taken from the up Crewe platform, with the site of Cheadle Hulme signal box on the down side.

https://www.railsigns.uk/info/clsig1.html says

'SDO' (optic diode) signals supplied by Ansaldo are used exclusively in the Manchester South area. This kind of signal head has two apertures, both of which are equipped to display three colours. In normal operation, they present a similar appearance to other twin-aperture signals except that a green aspect is displayed in the upper aperture. The lights are derived from single-filament lamps (one for each colour), directed through the aperture by dichroic mirrors. In the event of a lamp failure, a red, single yellow or green aspect will be displayed in the alternative aperture. For that reason, an SDO signal head always has two light units, even if the signal is only required to display two or three aspects.

That’s something I didn’t know until today, even though I see these signals all the time!

I have occasionally seen strange aspects on these signals, perhaps only transitory, but seemingly invalid or at least strange, such as red aspect with junction indicator lit.

 

[satisnek]

If you mean the attached (https://s0.geograph.org.uk/geophotos/04/52/43/4524388_e02f9eb4_800x800.jpg) they’re not LED 4 aspect signals. A “dichroic mirror signal head” apparently (https://www.networkrailmediacentre....g-system-commissioned-on-west-coast-main-line).
View attachment 181072
Ansaldo implementation, which failed completely for a number of days a few years ago (https://www.railforums.co.uk/threads/crewe-delays-16-sep.209188/), so not thought to be all that reliable.

Picture taken from the up Crewe platform, with the site of Cheadle Hulme signal box on the down side.

https://www.railsigns.uk/info/clsig1.html says

That’s something I didn’t know until today, even though I see these signals all the time!

I have occasionally seen strange aspects on these signals, perhaps only transitory, but seemingly invalid or at least strange, such as red aspect with junction indicator lit.

That's fascinating! I had always assumed that these were early LED signals. Seems like a bit of a sledgehammer to crack a walnut.

 

[edwin_m]

I have occasionally seen strange aspects on these signals, perhaps only transitory, but seemingly invalid or at least strange, such as red aspect with junction indicator lit.

This is actually relatively common with all types of signal.

The signal can't display a proceed aspect until the junction indicator is proved to be lit, as otherwise the driver might be misled into expecting a faster route straight ahead. So the junction indicator lights up slightly before.

In relay signalling the delay is just the response time of a relay in the cabinet next to the signal, so very short and may not be noticeable particularly with incandescent bulbs that take a fraction of a second to illuminate. But with computer-based interlockings the delay can be a second or so, as the detection has to go back to the interlocking when it next communicates with that module, and then the instruction to clear the signal has to follow. And if the route indicator lights correctly but due to some failure the system doesn't detect it, then it will stay lit with a red signal indefinitely.

 

[jfollows]

This is actually relatively common with all types of signal.

The signal can't display a proceed aspect until the junction indicator is proved to be lit, as otherwise the driver might be misled into expecting a faster route straight ahead. So the junction indicator lights up slightly before.

In relay signalling the delay is just the response time of a relay in the cabinet next to the signal, so very short and may not be noticeable particularly with incandescent bulbs that take a fraction of a second to illuminate. But with computer-based interlockings the delay can be a second or so, as the detection has to go back to the interlocking when it next communicates with that module, and then the instruction to clear the signal has to follow. And if the route indicator lights correctly but due to some failure the system doesn't detect it, then it will stay lit with a red signal indefinitely.

Yes, that makes sense, thank you.
I guess the thing is that the ‘wierd’ aspect I sometimes see doesn’t matter, because a train driver won’t see it.

 

[py_megapixel]

If you mean the attached (https://s0.geograph.org.uk/geophotos/04/52/43/4524388_e02f9eb4_800x800.jpg) they’re not LED 4 aspect signals. A “dichroic mirror signal head” apparently (https://www.networkrailmediacentre....g-system-commissioned-on-west-coast-main-line).
View attachment 181072
Ansaldo implementation, which failed completely for a number of days a few years ago (https://www.railforums.co.uk/threads/crewe-delays-16-sep.209188/), so not thought to be all that reliable.

Picture taken from the up Crewe platform, with the site of Cheadle Hulme signal box on the down side.

https://www.railsigns.uk/info/clsig1.html says

That’s something I didn’t know until today, even though I see these signals all the time!

I have occasionally seen strange aspects on these signals, perhaps only transitory, but seemingly invalid or at least strange, such as red aspect with junction indicator lit.

Thanks for that information. Yes I was aware they weren't LEDs, since when they change you can see the fading effect of the filaments, but I had no idea they were configured like that

 

[edwin_m]

Yes, that makes sense, thank you.
I guess the thing is that the ‘wierd’ aspect I sometimes see doesn’t matter, because a train driver won’t see it.

Train drivers might see it when waiting at or approaching a signal. But they will be used to it, and I don't believe modern systems display even momentarily an aspect that is not "between" the one displayed before and the one intended to be displayed. The transition through red when a mechanical searchlight signal was going from yellow to green was probably the most potentially scary for the uninitiated.

Rather early in the life of the Solid State Interlocking a tweak was made to the software because in an unusual situation arising from a lamp proving failure at the next signal, a signal could go to a proceed aspect for a short time before dropping back to red.

 

[MadMac]

On the approaches to London Victoria, on Grosvenor Railway Bridge, I seem to recall from the distant past three aspect signals with Y-R-Y, leading up to R-Y as the final signal before platform. Am I misremembering?

We converted the approach signals into Glasgow Central to that arrangement many years ago. In the wake of Moorgate, there was concern about giving a green into a terminal platform.

 

[GW43125]

If you mean the attached (https://s0.geograph.org.uk/geophotos/04/52/43/4524388_e02f9eb4_800x800.jpg) they’re not LED 4 aspect signals. A “dichroic mirror signal head” apparently (https://www.networkrailmediacentre....g-system-commissioned-on-west-coast-main-line).

I've also found these SDO signals interesting-if there's one lamp for each of the three colours then that must mean some chunky light modules-I had to take delivery of a modern Dorman light module once and that was bad enough!

 

[Taunton]

I have occasionally seen strange aspects on these signals, perhaps only transitory, but seemingly invalid or at least strange, such as red aspect with junction indicator lit.

Well semaphores could do the same.

Old GWR/WR installations quite commonly had Home and Distant on the same post. Home operated by cable from the adjacent signalbox, Distant by battery and electric motor from the next box ahead. They were Slotted together so as not to display wrong combinations, but when the box nearby put the Home back it would snap up with a clang, but the Distant would take several seconds to make its juddering way back. Take a photo at just the right moment and you could get Home on and Distant Off. There used to be a photo on the Internet of the old gantry at Taunton West showing exactly this, which I was familiar with, but it seems to have gone.

 

[midland1]

Well semaphores could do the same.

Old GWR/WR installations quite commonly had Home and Distant on the same post. Home operated by cable from the adjacent signalbox, Distant by battery and electric motor from the next box ahead. They were Slotted together so as not to display wrong combinations, but when the box nearby put the Home back it would snap up with a clang, but the Distant would take several seconds to make its juddering way back. Take a photo at just the right moment and you could get Home on and Distant Off. There used to be a photo on the Internet of the old gantry at Taunton West showing exactly this, which I was familiar with, but it seems to have gone.

I got a photo of this happening at a Castleford in Yorkshire about 20 years ago

== Doublepost prevention - post automatically merged: 1 Jun 2025 ==

If you mean the attached (https://s0.geograph.org.uk/geophotos/04/52/43/4524388_e02f9eb4_800x800.jpg) they’re not LED 4 aspect signals. A “dichroic mirror signal head” apparently (https://www.networkrailmediacentre....g-system-commissioned-on-west-coast-main-line).
View attachment 181072
Ansaldo implementation, which failed completely for a number of days a few years ago (https://www.railforums.co.uk/threads/crewe-delays-16-sep.209188/), so not thought to be all that reliable.

Picture taken from the up Crewe platform, with the site of Cheadle Hulme signal box on the down side.

https://www.railsigns.uk/info/clsig1.html says

That’s something I didn’t know until today, even though I see these signals all the time!

I have occasionally seen strange aspects on these signals, perhaps only transitory, but seemingly invalid or at least strange, such as red aspect with junction indicator lit.

When these were put it was pointed out that the red and yellow colours were very similar, could you have seen a yellow thinking it was a red.

 

[norbitonflyer]

I guess yellow-red-yellow is theoretically possible

Used at the penultimate signal before a terminus - as a green aspect would never be needed: the buffer stops are permanently red!

== Doublepost prevention - post automatically merged: 1 Jun 2025 ==

I have occasionally seen strange aspects on these signals, perhaps only transitory, but seemingly invalid or at least strange, such as red aspect with junction indicator lit.

That's quite normal - the feathers are designed to light up a moment before the signal clears from red. The ensures the feathers are lit when the proceed aspect appears, avoiding errors by a driver, or any possibility that the feathers fail to light but the proceed aspect does.

 

[jfollows]

That's quite normal - the feathers are designed to light up a moment before the signal clears from red. The ensures the feathers are lit when the proceed aspect appears, avoiding errors by a driver, or any possibility that the feathers fail to light but the proceed aspect does.

Thanks, yes, and I agree, but my recollection with the Ansaldo signals is that the moment between the two indications is a long one. I looked but can’t find the picture I took of a position 4 feather on its own at Alderley Edge, for a train being sent down the up line towards Wilmslow.

 

[dk1]

We have 3 aspect signals that cannot show a red on the approach to Diss (up) and on the approach to Europa Jcn and Boss Hall Jcn (both Ipswich) so yes, they can show preliminary caution.

 

[dan4291]

We have 3 aspect signals that cannot show a red on the approach to Diss (up) and on the approach to Europa Jcn and Boss Hall Jcn (both Ipswich) so yes, they can show preliminary caution.

Some on the ECML too that I sign, at Marshall Meadows just before the Scottish border and at Benton junction north of Newcastle. In both cases the signals change from 3 aspect before to 4 aspect afterwards.

 

[edwin_m]

Used at the penultimate signal before a terminus - as a green aspect would never be needed: the buffer stops are permanently red!

Yes I'm aware of that. My question, not fully answered yet, is whether Y-R-Y is always used, or R-Y-blank-Y is used when there are no space or sighting constraints. That would allow the red to be nearest to the driver.

 

[GW43125]

Yes I'm aware of that. My question, not fully answered yet, is whether Y-R-Y is always used, or R-Y-blank-Y is used when there are no space or sighting constraints. That would allow the red to be nearest to the driver.

Plenty of installations R-Y-blank-Y, couldn't name one off the top of my head but there's plenty around (or were until LEDification took hold)