Flashing Green Aspect on Colour Lights

[Kraken]

Can anyone help shed any light on what a flashing green aspect on a colour light tells a driver? I live quite near Tallington crossing and frequently see one on what I think is the Down Fast there. I would guess it’s something like indicating the next signal is double yellow but I’ve not managed to find anything. Certainly not something I remember being taught about at signalling school.

 

[centro-323]

Old leftover from 140mph testing. A flashing green was meant to indicate that the next signal was green (or possibly another flashing green?), giving an extra signal section before a potential red, and thus more braking time.

These days they aren't much use, but remain anyway.

 

[Watershed]

It's part of the ultimately unsuccessful efforts to raise the ECML linespeed to 140mph. Flashing green signals were to indicate that the next signal was a flashing or steady green, thus giving an additional signal's warning of a signal at danger (steady green, double yellow and yellow).

Unfortunately, it was deemed that lineside signals could not be reliably read at that speed and hence linespeeds were kept at the existing 125mph. This restriction remains to the present day, with the only linespeed above 125mph being on HS1, which has in-cab signalling (the French TVM430 system).

Britain is somewhat of an international outlier for allowing even 125mph without any form of in-cab signalling or automatic train protection.

The flashing greens will likely remain in place until Peterborough is resignalled.

 

[ainsworth74]

Britain is somewhat of an international outlier for allowing even 125mph without any form of in-cab signalling or automatic train protection.

We're a bit of an outlier for even allowing over 100mph without those sorts of things!

 

[Kraken]

Thank you all. I wasn’t aware of the 140mph proposal at all so that’s fascinating.

 

[GC class B1]

I remember mention of flashing double yellow signals on WCML in LMR days. I think they were intended for high speed junctions such as Colwich and we’re intended to indicate that the speed limit through the junction was higer than a single yellow. Can anyone advise please.

 

[6Gman]

I remember mention of flashing double yellow signals on WCML in LMR days. I think they were intended for high speed junctions such as Colwich and we’re intended to indicate that the speed limit through the junction was higer than a single yellow. Can anyone advise please.

I'm sure I noticed a flashing double yellow on the Up Fast north of Stafford a couple of years ago - can anyone remind me of its purpose? (If indeed I did see what I thought I saw)

 

[rd749249]

Flashing double yellows and single yellows are quite common. For example, you’ll see them the on great eastern and great western not far from the terminus stations. All it does is indicate that the diverging route ahead is set. A flashing single yellow will be followed by a single yellow plus junction indicator which may or may not step-up to a less restrictive aspect, so a red can follow. Speed doesn’t really come into it. You drive to what you see.

 

[rower40]

Part of the problem with the concept of a flashing green is the philosophical question "Is steady green now a restrictive aspect?" - and what should AWS indicate at a steady green?

 

[GC class B1]

Flashing double yellows and single yellows are quite common. For example, you’ll see them the on great eastern and great western not far from the terminus stations. All it does is indicate that the diverging route ahead is set. A flashing single yellow will be followed by a single yellow plus junction indicator which may or may not step-up to a less restrictive aspect, so a red can follow. Speed doesn’t really come into it. You drive to what you see.

Thank you for this information. I now seem to remember seeing a flashing yellow on GWR recently. I think it was for crossing to the Oxford line at Didcot.

 

[norbitonflyer]

Thank you all. I wasn’t aware of the 140mph proposal at all so that’s fascinating.

That is why Mark 4s and Class 91s were given the branding "InterCity 225" (that being the metric equivalent of 140mph). Mark 4s also have a narrower profile than mark3s so that tilt could be fitted at some future date to allow such speeds. (We're still waiting)

 

[hexagon789]

Britain is somewhat of an international outlier for allowing even 125mph without any form of in-cab signalling or automatic train protection.

I think it's because of the way it was implemented - HSTs were designed to be able to stop from 125 in the same distance a conventional train took to stop from 100. So the consideration then is primarily signal sighting.

In other countries, the increase to 200km/h involved fully upgrading the lines to suit, and providing increased braking distance either by means of a full cab signalling system as in Germany or by double blocking and with an in-cab warning system (signalisation pre-annoncé) in France for example.

 

[Bald Rick]

Re the flashing yellows. These act as a form of approach control to indicate to the driver that a specific diverging route off the main line at a diverging junction signal. It can only apply to one diverging route (where there is more than one diverging route at the signal) and that route must have the highest speed of the diverging routes from that signal.

The sequence is normally FYY, FY, then a steady Y at the junction signal (with route indicator) clearing up to a less restrictive aspect as the train approaches if the route ahead is set and clear. Flashing aspects will not be enabled in the sequence if the signal in advance of the junction signal is at danger for any reason; this was a principle established following the Colwich accident.

 

[CyrusWuff]

Part of the problem with the concept of a flashing green is the philosophical question "Is steady green now a restrictive aspect?" - and what should AWS indicate at a steady green?

That's an easy one as the instructions are still in the London North Eastern Sectional Appendix.

For trains not authorised to exceed 125mph, flashing greens are treated exactly the same as steady greens, and you'll get an AWS bell for both.

 

[Railsigns]

Re the flashing yellows. These act as a form of approach control to indicate to the driver that a specific diverging route off the main line at a diverging junction signal. It can only apply to one diverging route (where there is more than one diverging route at the signal) and that route must have the highest speed of the diverging routes from that signal.

The sequence is normally FYY, FY, then a steady Y at the junction signal (with route indicator) clearing up to a less restrictive aspect as the train approaches if the route ahead is set and clear. Flashing aspects will not be enabled in the sequence if the signal in advance of the junction signal is at danger for any reason; this was a principle established following the Colwich accident.

It is permissible for the same flashing yellow sequence to apply to more than one diverging route in certain circumstances.

It is also permitted to display flashing yellow aspects on approach to a diverging junction when the first signal beyond the junction signal is at danger, providing the required criteria are satisfied.

 

[Bald Rick]

It is permissible for the same flashing yellow sequence to apply to more than one diverging route in certain circumstances.

It is also permitted to display flashing yellow aspects on approach to a diverging junction when the first signal beyond the junction signal is at danger, providing the required criteria are satisfied.

I stand corrected.

Presumably one of the certain circumstances in the first instance is that the divergent speed is the same?

In the second instance, I was convinced this was banned post Colwich; what are the required criteria?

 

[Railsigns]

Presumably one of the certain circumstances in the first instance is that the divergent speed is the same?

In the second instance, I was convinced this was banned post Colwich; what are the required criteria?

In the first instance, the permissible speeds of the diverging routes at the junction must be within 10 mph of each other, and the routes must have similar characteristics.

The second instance is permitted where most trains stop on approach to the first signal beyond the junction because of a station etc., or where there's a high probability of the signal clearing before the train reaches it.

 

[MadMac]

On the flashing greens, I was under the impression that they were normally disabled and were only enabled for test runs or if there was a "record attempt" for want of a better expression. Did this change at some point?

 

[najaB]

That's an easy one as the instructions are still in the London North Eastern Sectional Appendix.

For trains not authorised to exceed 125mph, flashing greens are treated exactly the same as steady greens, and you'll get an AWS bell for both.

Yes, that's what the SA say but the question is if that is the correct option since a steady green is more restrictive than the preceding flashing green. So for a vehicle that is authorised to exceed 125mph it really should be a horn/buzzer.

 

[Bald Rick]

On the flashing greens, I was under the impression that they were normally disabled and were only enabled for test runs or if there was a "record attempt" for want of a better expression. Did this change at some point?

No they were always enabled.

 

[Falcon1200]

Britain is somewhat of an international outlier for allowing even 125mph without any form of in-cab signalling or automatic train protection.

Don't AWS, and much later TPWS, perform that function, albeit not continuously ?

 

[edwin_m]

Don't AWS, and much later TPWS, perform that function, albeit not continuously ?

AWS doesn't really because if the driver cancels it, as they have to do to proceed, the system does not apply any further speed supervision. TPWS sort of does but not comprehensively - and 125mph was authorised 25 years or so before TPWS came along.

 

[LNW-GW Joint]

I've never quite understood multiple greens as used in North America (with speeds much lower than 125mph).
What does 4 greens (one above the other) mean? Four clear sections ahead maybe?

I think you can also get a couple of vertical greens in German/Austrian signalling on the rectangular boards they use, seemingly to mimic the old semaphore aspects.
In fact, if I am remembering right, when I first used Paddington (early 60s), the colour lights for the first few miles could give you 2 vertical greens, again maybe mimicking the old semaphore setup of home/distant signals on one post.

 

[Falcon1200]

AWS doesn't really because if the driver cancels it, as they have to do to proceed, the system does not apply any further speed supervision. TPWS sort of does but not comprehensively - and 125mph was authorised 25 years or so before TPWS came along.

Fair points, but they are at least 'some form of in-cab signalling' ?!!

 

[hexagon789]

Fair points, but they are at least 'some form of in-cab signalling' ?!!

No because they don't repeat the signal aspects separately, all thevsunfkower shows is signal clear or signal not clear. Its up to the driver to remember exactly which "not clear" aspect was displayed at the signal. Also BR treated it more as an advisory system, it was permissible except in foggy conditions to continue at full linespeed with the AWS isolated due to a failure into at least the 1990s.

 

[LNW-GW Joint]

AWS doesn't really because if the driver cancels it, as they have to do to proceed, the system does not apply any further speed supervision. TPWS sort of does but not comprehensively - and 125mph was authorised 25 years or so before TPWS came along.

It's a curiosity that the ECML (and more recently parts of the MML and XC routes) can run at 125mph without speed supervision (unless TPWS+ does this?).
The GW route has its ATC, and the WCML has TASS above 110mph.
ETCS will solve this as it rolls out.

 

[najaB]

I've never quite understood multiple greens as used in North America (with speeds much lower than 125mph).
What does 4 greens (one above the other) mean? Four clear sections ahead maybe?

America uses speed signalling as opposed to route signalling. The details will differ depending on the railroad but multiple greens would usually mean to proceed at maximum allowed speed.

and the WCML has TASS above 110mph

I thought that TASS only controls tilt, rather than speed?

 

[GC class B1]

It's a curiosity that the ECML (and more recently parts of the MML and XC routes) can run at 125mph without speed supervision (unless TPWS+ does this?).
The GW route has its ATC, and the WCML has TASS above 110mph.
ETCS will solve this as it rolls out.

I understood that TASS was to permit 125 MPH when the tilt system was operative on the train and advised the driver to restrict the train speed on curves when Tilt was not operative and was not a signalling system.

 

[norbitonflyer]

No because they don't repeat the signal aspects separately, all thevsunfkower shows is signal clear or signal not clear. Its up to the driver to remember exactly which "not clear" aspect was displayed at the signal. Also BR treated it more as an advisory system, it was permissible except in foggy conditions to continue at full linespeed with the AWS isolated due to a failure into at least the 1990s.

It was a major factor in the Southall collision in 1997.

 

[LNW-GW Joint]

I thought that TASS only controls tilt, rather than speed?

Well I believe TASS stands for Tilt And Speed Supervision (above 110mph).
Reportedly, non-tilt stock will soon be running at up to 125mph on the WCML, without TASS.