Double yellow flashing aspects?

[MrsCake21]

Hi all,
I've read the rule book and it states that a double flashing yellow, followed by a single yellow flashing, and so on, indicates, you are coming to a diverging junction where the diverging route is of a slower speed.

Is this the case at all diverging junctions around the country?
Would there not be flashing yellows if the speed of the diverging junction were the same?
If there were several diverging routes at a junction all with slower speeds, would flashing yellows be used in approach to them all. (plus an indicator) ?

Finally,
I've heard it referred to as a, 'highest speed diverging route'. Isn't this a contradiction of what is happening.

Thanks

 

[jfollows]

See attached

For example, from page 74
Because flashing aspect sequences are generally applied to diverging signal routes with high permissible speed limits, the likelihood of a train exceeding the permissible speed limit would increase if the junction turnout speed is less than train drivers expect. Table 4 sets out permissible speed limit criteria typically applied to control this.

 

[jfollows]

The rules changed over time, generally to make flashing aspects more applicable than they used to be. Cheadle Hulme was 45mph diverging / 100mph straight, and remained MAR initially. It’s now MAY-FA4 even though the speed limits are unchanged. The version I posted above is relatively recent but may not be the most recent.

 

[LRV3004]

Some of it is dependent on route knowledge as well - for instance I sign Crewe to Daventry via Bescot, and the divergence at Aston South Junction onto the freight only section to Stechford does not have flashing yellows prior to it; in fact we usually get greens all the way and are relying on the correct indication being displayed on the signal at the end of Aston station platform. That said however, the line speed is only 30mph, dropping to 20 for the junction onto the line towards Stechford, so there would be time to stop if an incorrect route had been set.

 

[driver9000]

Finally,
I've heard it referred to as a, 'highest speed diverging route'. Isn't this a contradiction of what is happening.

No because there may be more than 1 diverging route at the junction. The lower speed turn outs would not be signalled using flashing aspects.

 

[Ditop23]

No because there may be more than 1 diverging route at the junction. The lower speed turn outs would not be signalled using flashing aspects

I think I'm misunderstanding the pont of flashing yellows.

I thought they were an extra precaution to make the driver slow down for the diverging junction as it's a slower speed.?

But you're saying there could be 2 diverging routes and the lowest speed would not have flashing yellows.
So therefore what's the point of the flashing yellows if its only to indicate for the highest speed route at a diverging route. ?

 

[jfollows]

They are for the opposite purpose, they are to inform the driver that the diverging route is set and clear, so the driver only needs to moderate the train’s speed appropriate to the diverging route speed. Without them, the train’s approach speed would be slower, regulated by adverse signal aspects.

Previously, for example, MAR would cause the train to be regulated almost to a stand by a red aspect in advance of the junction.
My example of Cheadle Hulme, for example (up direction with diverging route set towards Macclesfield), meant that trains almost came to a stand on the viaduct, when the signal would clear and the train would re-accelerate to 45mph for the diverging junction. Now it only has to slow down to 45mph from line speed by the time it reaches the junction.

Multiple diverging routes - only the highest speed diverging route gets flashing yellows. Drivers know which route this is and moderate the train’s speed accordingly.

(See https://www.railforums.co.uk/threads/colwich-junction-in-the-1980s-vs-today.221018/ for when things go wrong)

 

[GraphitProjekt]

I thought they were an extra precaution to make the driver slow down for the diverging junction as it's a slower speed.?

i think flashing yellows came about in the context where trains were either stepped down fully to red (approach control) - or at least passed through double yellow and single yellow as if they were approaching a red but it actually clears to a diverging route before they see it. Flashing yellows means you can go faster because the fact it's flashing means it's not actually a red signal your approaching but at least a yellow one with a diverging route set

 

[MarkyT]

Some of it is dependent on route knowledge as well - for instance I sign Crewe to Daventry via Bescot, and the divergence at Aston South Junction onto the freight only section to Stechford does not have flashing yellows prior to it; in fact we usually get greens all the way and are relying on the correct indication being displayed on the signal at the end of Aston station platform. That said however, the line speed is only 30mph, dropping to 20 for the junction onto the line towards Stechford, so there would be time to stop if an incorrect route had been set.

Divergences where the turnout route speed is no more than 10mph lower than the straight route do not normally require any form of approach release from red or yellow so are 'free aspect' depending only on the forward aspect state of signals on the selected route. There are some places where approach signalling wouldn't always give sufficient notice to the driver to pull up short of the junction if they'd been mis-routed and there was an applicable traffic restriction for the route actually set (weight, traction, loading gauge, knowledge, etc.). The preliminary route indicator was invented for such cases and displays an arrow in the same orientation as the upcoming Junction Indicator.

i think flashing yellows came about in the context where trains were either stepped down fully to red (approach control) - or at least passed through double yellow and single yellow as if they were approaching a red but it actually clears to a diverging route before they see it.

There's a junction control technique applied widely on the Southern Region called 'Free Yellow'. Applicable only in 4-aspect areas, this looks at the achievable speed at the junction signal if the straight route was set and the first signal beyond the junction was at red. If that speed is below the turnout speed then approach release from yellow can be applied for the turnout route without any advance notice from flashing yellows or splitting distants. A PRI might still be wise if either route beyond the divergence had any traffic restrictions however.

Flashing yellows means you can go faster because the fact it's flashing means it's not actually a red signal your approaching but at least a yellow one with a diverging route set

Approach release from red conditions often slow trains down far more than is strictly necessary for a turnout route and some layouts, after doing that, feature a long distance between the signal and the slow turnout so risk a confused driver accelerating significantly again and potentially speeding dangerously through the junction, as happened at Peterborough. ETCS is the answer, as permitted speed envelopes for junctions within movement authorities can be calculated and enforced on board far more optimally.

 

[PudseyBearHST]

If I could ask a related question, could someone please give me some examples of MAY-YY, approach on single yellow? Thanks

 

[JN114]

If I could ask a related question, could someone please give me some examples of MAY-YY, approach on single yellow? Thanks

A lot (most) have vanished in the past few years and replaced with MAY-FA

But in the not too distant past, Haywards Heath for the “loop” platforms; Stoats Nest Jn going southbound from the Down Fast to Down Redhill. I believe also Staines (towards Virginia Water) before recent resignalling.

 

[Tilting007]

Rugby TV junction is 125mph to both the Trent Valley and West Mids.
A PRI shows the way the route is set.

 

[PudseyBearHST]

A lot (most) have vanished in the past few years and replaced with MAY-FA

But in the not too distant past, Haywards Heath for the “loop” platforms; Stoats Nest Jn going southbound from the Down Fast to Down Redhill. I believe also Staines (towards Virginia Water) before recent resignalling.

Fascinating, thanks!

 

[MarkyT]

If I could ask a related question, could someone please give me some examples of MAY-YY, approach on single yellow? Thanks

That's the same as my description of 'free yellow' above, so must be the modern term for the arrangement. I can't think of a specific example, but they are common in the Southern Region, especially in the busy inner suburban area where they keep traffic moving smoothly through the many junctions.

 

[The Planner]

Rugby TV junction is 125mph to both the Trent Valley and West Mids.
A PRI shows the way the route is set.

Rugby has a more interssting flashing sequence now for both crossing at Hillmorton and for going into P1 on the down. You can't have overlapping sequences for obvious reaaons so Hillmorton can get a double flashing yellow, P1 a single.

 

[newtownmgr]

There are flashing yellows at St Andrews Jct leading up to Bordesley Jct in Birmingham. Although they operate both coming from Grand Jct (New St) and Landor St Jct (Saltley) they were primarily put in place for the latter to aid heavy freight trains coming up from Landor St not having to shut off power/brake etc whilst still on the bank up from Landor St. Line speeds are 15 from the new st direction, 25 from the Saltley direction on to 60 then 20 taking the jct at Bordesley. Any freight coming up from Landor st isn’t likely to get much above 25/30 between the junctions.

Back in BR days there was usually a bank engine (class25 or 31) provided at Saltley.

 

[The Planner]

There are flashing yellows at St Andrews Jct leading up to Bordesley Jct in Birmingham. Although they operate both coming from Grand Jct (New St) and Landor St Jct (Saltley) they were primarily put in place for the latter to aid heavy freight trains coming up from Landor St not having to shut off power/brake etc whilst still on the bank up from Landor St. Line speeds are 15 from the new st direction, 25 from the Saltley direction on to 60 then 20 taking the jct at Bordesley. Any freight coming up from Landor st isn’t likely to get much above 25/30 between the junctions.

Back in BR days there was usually a bank engine (class25 or 31) provided at Saltley.

That is just a normal sequence though and falls in the MAY-FY3 category above as they are just single flashing yellows.

 

[Annetts key]

A couple of additional points.

If there are multiple diverting routes at the junction or junctions ahead of a junction signal, the flashing aspect sequence normally only applies to one diverting route.

Hence:

• "Straight on" or highest speed route - no junction or route indication, normal aspect sequence.
• Highest speed diverging route, junction signal shows a single yellow with junction indicator and flashing aspect sequence leading up to it.
• All other diverging routes - approach release / approach control from red.

Note that the flashing aspect system is optional, it may not have been provided at a particular signal. With relay based interlocking, it is considerably more expensive. It is also relatively new in signalling terms, so will only feature on older signalling schemes if money was found to add it afterwards. Where not provided, the signal is approach released / approach controlled for all diverging routes. If any of the flashing aspect signals fail to flash, the system drops out of flashing aspect mode and the junction signal goes to red and operates in the approach release / approach control mode.

With a flashing yellow approach, the junction signal can be permitted to step up from a single yellow to a less restrictive aspect once the train has passed a certain point on approach to the junction signal. So if the line is clear ahead, it may step up to a green.

Similarly, a signal that is approach controlled / approach released will step up to the aspect that is appropriate for the diverging line and how far the line is clear ahead. When it steps up is determined by the train passing a certain point. In addition, there may be a time delay before the aspect changes. The length of the time delay may vary depending on which route has been set and the junction speed for that route. Lower speed routes having a longer time delay.

The train reaching the certain point may be detected by track circuits, axle counters or treadles (in combination with a track circuit).

The objective being to slow the train down to a suitable speed for the junction. However, due to the difference in performance of train braking systems, it is always a crude compromise on mixed traffic lines.

Where a time delay is also used, when the signal steps up then depends on the speed of the train.

 

[PudseyBearHST]

That's the same as my description of 'free yellow' above, so must be the modern term for the arrangement. I can't think of a specific example, but they are common in the Southern Region, especially in the busy inner suburban area where they keep traffic moving smoothly through the many junctions.

Thanks

What are some of the more quirkier locations with flashing yellow sequences?
E.g. are there any to a bay platform, which ones have the lowest speed divergences, which ones have flashing yellows onto a theatre box indication as oppose to a junction indicator, which has flashing yellows for the most diverging routes etc…

Are there locations where flashing yellows would only activate if the signal after the junction is showing proceed otherwise it would only ever be approach controlled?

Thanks

 

[driver9000]

There is also an interesting somewhat non standard arrangement spacing Carstairs on the Down. Trains routed to the South curve and ultimately Edinburgh can receive the usual flashing yellows approach but trains can be routed to the Down Passenger Loop opposite the station with a steady YY followed by a flashing Y at Carstairs South Junction then the signal after the junction will have the Junction Indicator for the loop. This was done to mitigate against Driver confusion over routes offered.

A similar set up applies north of Person for trains being routed into the Up Passenger Loop at Oxheys but that's related to restricted sighting of the signal that would be flashing YY rather than 2 routes using flashing aspects close together.

 

[JN114]

are there any to a bay platform

Been a long time since I looked in any detail, but High Wycombe bay used to be MAY-FA

 

[The Planner]

Been a long time since I looked in any detail, but High Wycombe bay used to be MAY-FA

Still is.

 

[Tilting007]

Rugby has a more interssting flashing sequence now for both crossing at Hillmorton and for going into P1 on the down. You can't have overlapping sequences for obvious reaaons so Hillmorton can get a double flashing yellow, P1 a single.

Shame the speed into 1 had to be reduced, although understandable given the risk

 

[The Planner]

Shame the speed into 1 had to be reduced, although understandable given the risk

Still gives a much better approach timetable wise.

 

[Tilting007]

Still gives a much better approach timetable wise.

Of course. Big difference.

 

[PudseyBearHST]

Been a long time since I looked in any detail, but High Wycombe bay used to be MAY-FA

Thank you