Collision and derailment near Salisbury (Fisherton Tunnel) 31/10/21

[Stigy]

Most likely 158884/158883 which were in Southampton bay all week.

It says Cross Country?

 

[Tomnick]

It could be. The interlocking clears the signal in advance through the junction before the one in rear clears, even on the straight route. However, the approach control works for trains routed straight on from one signal to the next, when coming to a stand at the conflicting junction. I.e. Red - approach controlled (Y) - Red. Diverging at the junction, coming from Nottingham I could understand it with a line speed of 15mph through the turnout, but trains towards Grantham with a higher line speed? As this is a newer installation than probably the installation at Salisbury then it made me wonder why a similar approach controlled arrangement isn't in place. Perhaps the RAIB report will offer some insight into this?

(I was on early turn. Gone home by the time you passed).

That all makes sense. In simple terms, I'd imagine that the condition for the first signal to clear is either second signal cleared or track circuit in rear occupied for X seconds - thus, when you put the route in with no train immediately approaching, the first signal won't clear until the second does. I guess that the risk otherwise is that you pass the distant at Y, see the first stop signal also at Y along the straight, and over-estimate the short distance to the R.

The arrangement at Salisbury is, I understand, not permitted or at least strongly discouraged nowadays - more on that in my reply to the below...

(there are worse places to be stopped for 40 minutes than at your first signal!)

As a layman in terms of signalling, I would appreciate some explanation as follows. If a driver sees a double yellow, does he brake to pass the following yellow (assuming it is still yellow) at say half the speed he approached the double yellow so there is plenty of distance to stop and leave allowance for a higher braking rate if necessary. Is there some rule of thumb for when to brake and at what rate or is it all a matter of route knowledge. My understanding is that the specific rolling stock brake performance requirement is based on stopping in Full Service within signal distances ( less an allowance) in accordance with GM/RT2045.

As I understand it, from what you are saying being able to stop in the distance between the double yellow and yellow signals, I.e perhaps only half the distance to the likely red signal is not preferable. Have I misunderstood what you mean. How much advance warning of a red signal do you believe a driver should have for the conditions. i.e gradient and line speed.

i am aware that defensive driving is good practice and using Full Service braking is not good practice In normal circumstances.

The problem with the arrangement approaching SY31 is that the double yellow is a long way from the red, and the single yellow is relatively close to it. SY29R is a three-aspect distant at braking distance from SY29, which shows YY when SY29 is Y and SY31 at R. The risk is that you recognise that it's a really long way to the red and don't brake immediately, then (for whatever reason) don't start braking until the single yellow comes into view, by which time it's probably too late. The problem is that there's too much braking distance! Personally I find it wise to get the brake in straight away to at least knock some of the speed off, that positive action also reinforcing the fact that you're on restrictive aspects - so you're less likely to forget that you need to brake for the red, and are in a better position if you do forget!

 

[DelW]

A simple derailing trap wouldn't be acceptable but a short 'overlap spur' leading to a friction bufferstop or in the old days a sand drag might be an option. Normally these are only provided where there is no room for any kind of sensible clear overlap, yet flexibility demands the ability to run towards the signal while the junction ahead is blocked. I would suggest treating the existing overlap as 'restricted', with an additional 'warning' class route from SY29 to SY31, enforcing a delayed yellow clearance at SY29. The unrestricted 'main' route would also still be available but would have an extended overlap through the junction. Whether the warner could have reduced speed further back sufficient to prevent the slide past, or limit its extent enough to avoid the collision, I really couldn't say. I want to reiterate this is a 40-year-old layout in this form. If it was inherently dangerous, surely that would have been uncovered before. Have there been any worrying SPADs at this junction in the past, or any near misses?

Thanks for the insight. As you say it's apparently been quite safe for forty years - which suggests it wouldn't be justifiable to spend extra money on all such locations just to reduce an already very low risk.

 

[Nicholas Lewis]

The problem with the arrangement approaching SY31 is that the double yellow is a long way from the red, and the single yellow is relatively close to it. SY29R is a three-aspect distant at braking distance from SY29, which shows YY when SY29 is Y and SY31 at R. The risk is that you recognise that it's a really long way to the red and don't brake immediately, then (for whatever reason) don't start braking until the single yellow comes into view, by which time it's probably too late. The problem is that there's too much braking distance! Personally I find it wise to get the brake in straight away to at least knock some of the speed off, that positive action also reinforcing the fact that you're on restrictive aspects - so you're less likely to forget that you need to brake for the red, and are in a better position if you do forget!

Indeed being overbraked is as bad as being underbraked for reasons you describe. However, the RAIB initial report doesn't cite 1L53 as having a TPWS intervention before it arrived at SY31 TSS so it speed would have been below the previous two OSS indicating the trains speed was under control.

 

[swt_passenger]

It says Cross Country?

Was probably accidental - but it did run roughly in the normal XC path to Reading though, ie 0916 from Southampton P1, a bit earlier from P5.

 

[Taunton]

trains have to be braking continuously from the outer distant at YY to even hit the start of the 50mph limit correctly.

I sincerely hope not! The outer distant at Yellow means SY29 - roughly at the 50 limit - is Red.

YY means double yellow.

But your comment does make one think, what if SY29R had been single yellow because of a movement off the Laverstock curve. I don't believe it has a further outer distant.

Can't read the full report, but the summary doesn't mention adhesion problems as a factor.

It is about bending speed limits just to suit a non-standard overlap.

 

[Tomnick]

YY means double yellow.

But your comment does make one think, what if SY29R had been single yellow because of a movement off the Laverstock curve. I don't believe it has a further outer distant.

There is braking distance from SY29R to SY29, hence the discussion about it being over-braked to SY31 - although, thankfully, that doesn't appear to have been a factor.

 

[MarkyT]

That all makes sense. In simple terms, I'd imagine that the condition for the first signal to clear is either second signal cleared or track circuit in rear occupied for X seconds - thus, when you put the route in with no train immediately approaching, the first signal won't clear until the second does. I guess that the risk otherwise is that you pass the distant at Y, see the first stop signal also at Y along the straight, and over-estimate the short distance to the R.

The arrangement at Salisbury is, I understand, not permitted or at least strongly discouraged nowadays - more on that in my reply to the below...

(there are worse places to be stopped for 40 minutes than at your first signal!)

The problem with the arrangement approaching SY31 is that the double yellow is a long way from the red, and the single yellow is relatively close to it. SY29R is a three-aspect distant at braking distance from SY29, which shows YY when SY29 is Y and SY31 at R. The risk is that you recognise that it's a really long way to the red and don't brake immediately, then (for whatever reason) don't start braking until the single yellow comes into view, by which time it's probably too late. The problem is that there's too much braking distance! Personally I find it wise to get the brake in straight away to at least knock some of the speed off, that positive action also reinforcing the fact that you're on restrictive aspects - so you're less likely to forget that you need to brake for the red, and are in a better position if you do forget!

That overbraked 4-aspect arrangement when approaching a junction or station after a long section was very common in old mechanical layouts where the frame was retained for interlocking but semaphores were replaced by colour lights under TCB. It persisted in low-cost small schemes for decades afterwards all over the UK. It technically wouldn't break the 150% overall overbraking limit from the YY which was allowed under the old principle although is frowned on for 'drivability' in new schemes. A second intermediate distant signal might be provided today so in all cases the first caution encountered for stopping at either junction signal in a similar layout would be a double yellow.

YY means double yellow.

But your comment does make one think, what if SY29R had been single yellow because of a movement off the Laverstock curve. I don't believe it has a further outer distant.

No additional outer distant I believe although I don't know what the new interface with the resignalled Basingstoke area around Grateley looks like.

It is about bending speed limits just to suit a non-standard overlap.

The right-hand curve starting at Laverstock N Jn is about 600m radius, definitely requiring some reduction in speed from the 90mph approach. The restriction probably also conditions speed for reduced braking distances and headway compression between Tunnel Jn and the station required to accomodate the combined higher density service merging from both lines. Starts at a nicely memorable location, the 82 milepost. Not particularly risky in itself, especially as the PSR is enforced by its own permanently activated overspeed TPWS installation just before the first junction and a dedicated preceeding AWS magnet at the advanced warning board on approach (both becasue it is a third or more reduction). As I said earlier, the shortened overlap allows standage of a 9-car cl.159 at SY31 clear of Laverstock N Jn.

== Doublepost prevention - post automatically merged: 8 Nov 2021 ==

Rather than mitigating the consequences, wouldn't addressing the root cause be far better? In other words, cut the trees down.

Lets call it 'managing the vegetation' though.

 

[yorkie]

Rather than mitigating the consequences, wouldn't addressing the root cause be far better? In other words, cut the trees down.

Lets call it 'managing the vegetation' though.

There is a dedicated thread for that: Network Rail's vegetation problem


Edit: The thread to discuss the actual collision is closed until we have any further developments.

Below is a list of spin-off / related threads (this list is not necessarily exhaustive!):

• Signal spacing and where 4 aspects are needed
• Can fireworks cause poor railhead conditions?
• Network Rail's vegetation problem
• West of England line current service provision
• Do heavy locos cope with poor railhead conditions better than lightweight units?
• Was the Cowden crash due to cost cutting?
• Unnerving experience on 1803 from Paddington
• Should drivers instruct passengers to head to the rear of the train in the event of an impending collision?
• Communication to drivers should be improved
• Do class 158's/159's have forward facing cameras?
• Should we have a universal move to in-cab signalling?
• How common are serious wheel slide incidents?
• Could a train deploy an emergency dragging anchor of some sort, or a dragging wheel on the ballast?
• Trains without passenger access between units serving routes including tunnels
• Class 158/159 recovery following Salisbury collision
• Could alterations be made to increase stopping distance at critical locations?
• Track reconstruction around Fisherton Tunnel / Salisbury Tunnel Junction

 

[ainsworth74]

The RAIB today have published their interim report on this accident. The thread has therefore been unlocked to allow further discussion at this time.

The interim report can be found here and an extract can be found below:

Summary​

1 At around 18:43 hrs on 31 October 2021, train reporting number 1L53, the 17:20 hrs South Western Railway passenger service from London Waterloo to Honiton, collided with the side of train 1F30, the 17:08 hrs Great Western Railway passenger service from Portsmouth Harbour to Bristol Temple Meads. The collision took place at Salisbury Tunnel Junction, which is on the immediate approach to Fisherton tunnel, near Salisbury in Wiltshire.

2 The impact of the collision caused the front two coaches of train 1L53 and the rear two coaches of train 1F30 to derail. Both trains continued some distance into Fisherton tunnel following the collision, before they came to a stop. Thirteen passengers and one member of railway staff required treatment in hospital as a result of the accident, which also caused significant damage to the trains and railway infrastructure involved.

3 Train 1L53 approached Salisbury Tunnel Junction on the down main line. The movement of train 1F30 across the junction was being protected from trains approaching on this line by signal SY31, which was at danger (displaying a red aspect). Train 1L53 passed this signal, while it was at danger, by 191 metres, before colliding with train 1F30.

...

The accident​

...

21 At 18:41:09 hrs, 1 minute and 47 seconds before the accident (timings in this section are taken from the train’s OTDR, adjusted to synchronise with the time recorded by the signalling system), the driver of train 1L53 shut off power and allowed the train to coast down the prevailing 1 in 169 gradient. Ten seconds later he acknowledged the AWS warning for signal SY29R, which was showing a double yellow preliminary caution aspect. At 18:42:03 hrs, around 1,600 metres after passing signal SY29R and with the train travelling at 86 mph (138 km/h) on level track, the driver made a step 2 brake application. This was in accordance with his usual practice and was done with the intention of being able to stop the train at signal SY31, which was at this point around 1,500 metres away. Although the train’s speed began to reduce, analysis of OTDR data shows that its wheels began to slide almost immediately after this brake application was made. The driver made a full-service brake application five seconds later and moved the brake controller to the emergency brake position after a further six seconds. The train’s wheel slip/slide prevention (WSP) system was active throughout this braking, but the train’s speed reduced only slowly.

22 As the train approached signal SY31, which remained at danger, the Train Protection and Warning System (TPWS) fitted to the train detected that it was travelling above the set speed of the overspeed sensor (OSS) system fitted on approach to the signal, 34.5 mph (55.5 km/h). The TPWS system therefore made an emergency brake demand. This had no effect on the degree of braking demanded, because the maximum available braking had already been applied by the driver.

23 Train 1F27 passed across and clear of Salisbury Tunnel Junction less than 40 seconds before train 1L53 arrived. Subsequently, the driver of train 1L53, which was still sliding and rapidly approaching signal SY31, saw train 1F30 appear from the left and move into the path of his train. Train 1F30 was at this point travelling at 20 mph (32 km/h).

24 At the point where the two trains collided, train 1L53 was travelling at between about 52 and 56 mph (84 and 90 km/h). Train 1L53 struck 1F30 on its right-hand side near the front of the fourth coach, as both trains crossed the junction and as 1F30 was entering Fisherton tunnel.

...

Initial findings​

Railhead condition​

...

36 The railhead was found to be contaminated throughout the areas surveyed with a black deposit. This deposit consisted of leaf material which had been crushed under the wheels of passing trains and which is often associated with low adhesion conditions. Analysis showed that many areas had a medium or heavy level of contamination, and that the thickness of the leaf deposit was relatively consistent through each of the areas surveyed. It is likely that the rails were wet at the time of the accident. Wet values of the coefficient of friction measured as part of the post-accident survey were found to be between 0.2 and 0.02, suggesting that there was low friction between the wheels and the railhead. The average rate of deceleration of train 1L53 suggests adhesion levels closer to the 0.02 value were prevalent at the time of the accident. Samples taken from the area around signal SY29 and between there and the junction showed that the deposits were smeared and flaky and typical of the sort of contamination layers which exist after train wheels have slid over them.

Train driving​

37 OTDR data shows that train 1L53 was travelling within the permitted speed as the train approached Salisbury Tunnel Junction. RAIB’s investigation will continue to consider the driver’s actions and factors that may have influenced them, including the industry’s arrangements for advising drivers about poor adhesion conditions and the guidance and instructions relating to reporting low adhesion and driving in such conditions.

Signalling​

38 Evidence indicates that the signalling system was operating as designed before the accident. Signal SY31 was at danger as train 1L53 approached it, and the routing of trains across Salisbury Tunnel Junction on the day of the accident was in accordance with normal signalling practice.

Management of low adhesion​

...

43 On the day of the accident (a Sunday), the RHTT that was due to treat Salisbury Tunnel Junction on the down main line at 17:03 hrs had not yet passed through the area because it had been rescheduled. This rescheduling meant that it would (but for the accident) have passed the junction at around 23:00 hrs. The revised schedule, which was a consequence of planned engineering work between Southampton and Brockenhurst, meant that there would have been an interval of 36 hours between the RHTT runs over the weekend.

44 The last RHTT to run over the junction passed over it at 11:06 hrs on 30 October, around 32 hours before the accident occurred. This RHTT delivered highpressure water jetting, and although it could apply adhesion modifier gel, it was not required to do so on this section of track.

...

 

[VEP3417]

description of what happens starts at point 21 to 24 of the document but basically it says one train braked for a signal and suffered wheel slip and continued passed the signal and collided with the other train if ive understood it correctly

 

[John R]

Another key aspect appears to be that the RHTT was scheduled to run twice daily during the week to ensure that there was at least one run in any 24 hour period. However, at weekends it was only scheduled to run one a day. (Why? Is leaf fall less of a problem at weekends?)

Unfortunately, on the day in question, the trip was rescheduled, and by the time of the accident it was 32 hours since the last passage of the train (so well outside the 24 hour requirement). The report also notes that it found that railhead conditions were such that very low adhesion was likely to have been prevalent.

 

[bramling]

Can’t help but wonder if the fact braking did not commence for 1,600 metres after the double yellow aspect will become a factor in the final report. No doubt not an issue normally, but evidently an issue during autumn, and probably not helped by being a 3-car. This signal setup must qualify as being “over-braked”, which may have been something touched on during previous discussions here.

Also notable the speed at point of collision was quite high. The rolling stock clearly performed well in the circumstances. This incident could have been very much worse.

 

[John R]

Can’t help but wonder if the fact braking did not commence for 1,600 metres after the double yellow aspect will become a factor in the final report. No doubt not an issue normally, but evidently an issue during autumn, and probably not helped by being a 3-car.

Also notable the speed at point of collision was quite high. The rolling stock clearly performed well in the circumstances. This incident could have been very much worse.

I think there's an implication in the first comment the type of which I think we should avoid.

As for the second point, it probably helped that the closing speed was 20mph lower, and it was a glancing blow. If the SWR train had been around 40 seconds earlier it would have collided with the southbound GWR service, and that would clearly have been a much more severe impact.

 

[bramling]

I think there's an implication in the first comment the type of which I think we should avoid.

It is what it is, unfortunately. But I do think *how* this came about will prove quite important to the final verdict. Method of braking was also cited in the Stonegate report, which was also an adhesion-related incident.

As for the second point, it probably helped that the closing speed was 20mph lower, and it was a glancing blow. If the SWR train had been around 40 seconds earlier it would have collided with the southbound GWR service, and that would clearly have been a much more severe impact.

Agreed.

 

[Mcr Warrior]

Apologies for asking a question that has possibly already been answered upthread or in the interim RAIB report, which I've not yet been able to successfully download, but was there some intermediate signal SY30 (?) located after SY29R, if so, how much nearer is that to the junction, and what would it have been displaying?

Single yellow? Would the the train (1L53) have been braking by then, and how fast was it travelling at that particular point?

And ought the signals have been set so that the train that was involved should instead have been stopped at SY30, further away from the Junction, rather than at the nearer SY31?

 

[Taunton]

It does appear there was an attitude at NR that running the RHTT was somehow optional, probably a nuisance item in the budget, and a lack of understanding that it was run for safety reasons.

I do wish that RAIB were more penetrating in their questions. The RHTT ran a half service at weekends (and then didn't run because of works, which surely should be known to happen mainly at weekends). Why indeed. What is it that means you only need half treatment at weekends. Why, NR, why?

 

[Dieseldriver]

Apologies for asking a question that has possibly already been answered upthread or in the interim RAIB report, which I've not yet been able to successfully download, but was there some intermediate signal SY30 (?) located after SY29R, if so, how much nearer is that to the junction, and what would it have been displaying?

Single yellow? Would the the train (1L53) have been braking by then, and how fast was it travelling at that particular point?

And ought the signals have been set so that the train that was involved should instead have been stopped at SY30, further away from the Junction, rather than at the nearer SY31?

Sequence was
Double yellow at SY29R (signal is a distant signal capable of displaying a green, double yellow or single yellow).
Single yellow at SY29 (capable of displaying a green, single yellow or red).
Red at SY31 (the protecting signal for the junction, capable of displaying a green, single yellow or red).

 

[bramling]

Apologies for asking a question that has possibly already been answered upthread or in the interim RAIB report, which I've not yet been able to successfully download, but was there some intermediate signal SY30 (?) located after SY29R, if so, how much nearer is that to the junction, and what would it have been displaying?

Single yellow? Would the the train (1L53) have been braking by then, and how fast was it travelling at that particular point?

And ought the signals have been set so that the train that was involved should instead have been stopped at SY30, further away from the Junction, rather than at the nearer SY31?

The SWR train would have encountered SY29R displaying double yellow, SY29 displaying single yellow, then reaching SY31 at red.

What is notable is that SY29R provided first warning for two different stop signals depending on whether it was displaying double yellow or single yellow. Based on the RAIB interim report, the SWR train does not appear to have taken advantage of this early warning.

I’d be interested to know what SWR’s PDP guidance is. In the case of Stonegate, the PDP guidance seems to have been a factor in itself.

 

[swt_passenger]

Sequence was
Double yellow at SY29R (signal is a distant signal capable of displaying a green, double yellow or single yellow).
Single yellow at SY29 (capable of displaying a green, single yellow or red).
Red at SY31 (the protecting signal for the junction, capable of displaying a green, single yellow or red).

If it helps people get their bearings, post #372 also has reasonably detailed signalling diagrams for the area.

 

[ainsworth74]

It does appear there was an attitude at NR that running the RHTT was somehow optional, probably a nuisance item in the budget, and a lack of understanding that it was run for safety reasons.

Not quite sure how you've reached that conclusion from what's in the interim report. Sounds like your own opinion formed from pre-existing prejudices rather than evidence from the report itself to be honest.

I do wish that RAIB were more penetrating in their questions. The RHTT ran a half service at weekends (and then didn't run because of works, which surely should be known to happen mainly at weekends). Why indeed. What is it that means you only need half treatment at weekends. Why, NR, why?

I'm also puzzled how you can conclude that the RAIB won't be asking these questions considering this is the interim rather than final report and that for their final report one of the areas of further investigation will be:

Consider Network Rail’s policies and processes relating to low wheel/rail adhesion and how it managed the risk of low adhesion in the Salisbury area, including:

• Risk assessment and management systems
• Collation, analysis and dissemination of data
• Railhead treatment
• Monitoring of railhead conditions and information disseminated to operators
• Regulation of trains at high-risk locations o The management of vegetation

 

[Daytona]

Just a layman's view...

So I guess it would have required closer to 3 miles to stop from 86mph, starting a mile before the double yellow...

The max speed for the conditions would look to be significantly below the 95mph line speed. Closer to 60mph plus whatever safety margin they use. And that's with a driver braking from the double yellow.

So the track looks unserviceable for the schedule ?

Just wondering if the latest trains can feed back track conditions in real time ? ETCS ?

 

[rebmcr]

It will be interesting to see RAIB's conclusion about:

Consider the actions of the driver of 1L53 and any factors which may have influenced them

It may be the case that a Railway Emergency Call could have stopped 1F30 in time and prevented a collision, but at the same time, such a REC could very well have caused 1F27 to remain in the path of 1L53 instead.

It's a difficult question to answer, and perhaps RAIB and/or ORR should commission detailed modelling of various real-life and hypothetical incidents, to find out which course of action (REC vs no REC) carries the best overall probability of safety, in order to accurately inform TOCs' driving policies.

 

[Mcr Warrior]

Sequence was
Double yellow at SY29R (signal is a distant signal capable of displaying a green, double yellow or single yellow).
Single yellow at SY29 (capable of displaying a green, single yellow or red).
Red at SY31 (the protecting signal for the junction, capable of displaying a green, single yellow or red).

Thanks for clarifying.

If it helps people get their bearings, post #372 also has reasonably detailed signalling diagrams for the area.

Yes, helpful also. On page 13 of this thread. Not sure if it shows the relative distances.

P.S. What, if anything, does SY202 do?

 

[PG]

Full respect to the driver of 1L53 for remaining at the controls while approaching a, by then, unavoidable collision at the speed mentioned in the RAIB interim report.

 

[bramling]

Thanks for clarifying.


Yes, helpful also. On page 13 of this thread. Not sure if it shows the relative distances.

P.S. What, if anything, does SY202 do?

SY202 isn’t relevant to the collision, however it allows a train to berth at SY31 and reverse towards Andover via no.101 crossover (Laverstock Nth Junction).

 

[Annetts key]

Apologies for asking a question that has possibly already been answered upthread or in the interim RAIB report, which I've not yet been able to successfully download, but was there some intermediate signal SY30 (?) located after SY29R, if so, how much nearer is that to the junction, and what would it have been displaying?

Single yellow? Would the the train (1L53) have been braking by then, and how fast was it travelling at that particular point?

And ought the signals have been set so that the train that was involved should instead have been stopped at SY30, further away from the Junction, rather than at the nearer SY31?

Down Main signal sequence

SY29R
SY29
SY31

SY30 is an up direction signal on the Up Main.

This is typical numbering, with odd numbers used for one line and even numbers used for the other line.

 

[Mcr Warrior]

Down Main signal sequence

SY29R
SY29
SY31

SY30 is an up direction signal on the Up Main.

This is typical numbering, with odd numbers used for one line and even numbers used for the other line.

That's clear. So, what are their relative distances from Salisbury Tunnel Junction?

 

[Taunton]

Not quite sure how you've reached that conclusion from what's in the interim report.

Quite straightforward actually. Only schedule half the RHTT service on days when it is inconvenient (although a full timetable is being run). And then postpone it further. Why was the treatment not rescheduled to before the Sunday disruption from the works, rather than after? Why was the RHTT not positioned beyond the works before they began?

Keeping hammering on about "interim" is a bit of a diversion. It's a common string of pushing things away. Comment on the accident? "No, must wait for the report". Here's the report, comment on that? "No, it's only an interim report". Here comes the final report a year or so later. "Oh, procedures have changed since then, so it's no longer relevant".

The RAIB put enough resources into this. These key questions should have been getting put to NR management the Monday morning afterwards (and thus could appear in the interim). It would happen in any other business.

 

[swt_passenger]

That's clear. So, what are their relative distances from Salisbury Tunnel Junction?

Para 14:
“Signalling in the area is controlled from a signal box at Salisbury station. Signal SY31 protects the junction for trains approaching on the down main line. This is a three-aspect colour light signal, located around 191 metres from the point where the collision occurred. Before reaching signal SY31, trains approaching the junction on the down main line pass signal SY29R and signal SY29, which are respectively 3,314 metres and 978 metres from the point of collision.”
I think for your question you can assume the point of collision is very similar to the junction position?