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Incident at Chalfont & Latimer (21/06/20)

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SlimJim1694

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Let me start with the simple things: the cancellation reason that shows on RTT is the first that goes into TRUST. If it is subsequently corrected then that does not carry through. This is an artefact of how the feeds 'take' data from TRUST.

The way the NR train describer works between Mantles Wood and Harrow-on-the-Hill South Jn, where Marylebone IECC fringes with the Metropolitan Line, is a series of several queue berths. These queue berths work on the basis that when the train steps into the Met Line at, say, Mantles Wood then it enters the queue and steps forward as far as possible. If it is the one and only train moving on the southbound Met, then it steps up to the berth just prior to the first step back in. It works likewise for the northbound. This makes the train describer 'blind' to what happens on the Metropolitan Line.

For Harrow-on-the-Hill northbound departures and Amersham southbound departures, there is a berth offset that is a positive number (it's normally negative) against the fringing NR signal into the Metropolitan Line. This means that you will always have a constant dwell time at those stations in those directions. It works similiarly with a negative arrival for Harrow-on-the-Hill southbound and Amersham northbound.

Once you are on the Metropolitan Line proper, Network Rail systems are effectively blind to what is happening. Up until about October last year when it broke properly for the final time, RTT had a mechanism to use track circuit data pulled from Trackernet to track trains through the area. In a similar mechanism to how it works on Network Rail metal, it used the time it stepped into the circuits to infer a departure and arrival time at each location based on fixed offsets.

Chiltern introduced the GPS reporting facility onto their fleet more recently which meant I was less inclined to fix the issues around Trackernet as it, really, didn't seem worth it. GPS reporting works mildly different for each operator due to the whims of the systems that drive it but, in a simplified/generalised view, the data goes into Network Rail's GPS Gateway as well as some other data warehouses. There are a series of geofences at most reporting locations which translates from time/position to reporting. I don't have a view of how big those geofences are for the Metropolitan Line but for some locations they are insanely large and they also don't always require the train to stop in the station limits to report for a station call.

My personal opinion is that the geofence, based on what I know historically, is that the geofence for Chalfont & Latimer is ... large.
Thanks Tom. That's an extremely informative response and opens my eyes to how it all works. I'll be honest that up until reading that I wasn't really sure.


Just an aside... is there AWS on that LU section for the Chilterns?
 
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Deepgreen

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I have no idea what happened here but several worrying things about this:
A: I wonder what speed the crossover was taken in order for the 165 to come to a rest so close to the S8
B: was the move signalled
C: why were the points set to cross the 165 from the down to the up with another train occupying the platform.

Obviously goes without saying but i hope both drivers are okay after such an incident. I know crash standards have improved but i dont think underground trains deal well being shunted by mainline ones...
As has already been very well established, there was no contact at all between the two trains so crash-resistance, injury, etc., are not issues here. I imagine, though, that the two trains have very similar crash-resistance properties, as the 'tube' train was of the larger, surface roilling stock size, weight, etc.
 

Nym

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I don't believe they completely dump the entirety of the reservoirs - on older stock they'd dump the brake pipe, but with 'newer' stock (since 1973) with electronic brake control, there's no brake pipe from which to dump the air from, whilst dumping air from the reservoirs would reduce braking capability!

With London Underground at least, there's a very specific process to be followed 'post trip' as detailed on this page (with caveat that the info on that page may be outdated) which involves contacting control and an automatic speed limiter. Needless to say that if what's been hinted at upthread is true, expect the RAIB report to focus a lot on procedures and controls around what happens 'post trip'

The first ECEB (Electrically Controlled Emergency Brake) units used a small reservoir with three pressure switches attached (2ea for 2off safety circuits and 1 for a different safety circuit, TMS and SCAT Control), when the pressure in the resevoir (fed via a choke) fell below a qualified pressure, this would open the Safety Brake Circuit and Round Train Circuit.

SCAT would then disable the "Full Speed" Safety Brake Circuit for 180 seconds after the tripcock had been reset.

The Safety Brake Circuit would then control the power to the westcode valves on each car (and the emergency brake release control switch) such that it would drop out every proportional valve in the 7 step, and the rate valve, and cut the supply to the emergency brake release valves.

Only the "Slow Speed" one works while SCAT is in progress.

This is usually controlled either by a timer relay, part of the TMS, or something in an obscure black box (in the case of modern stocks).

Where smart valves are used with continuous brake control, the "Safety Brake" is an input into each Brake Gateway Unit (BGU) of which there is usually two per car, sometimes one.

Thanks Tom. That's an extremely informative response and opens my eyes to how it all works. I'll be honest that up until reading that I wasn't really sure.


Just an aside... is there AWS on that LU section for the Chilterns?
No, Train Stops only.
 

Surreytraveller

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This section has 4-aspect signalling, and the additional CBTC aspect will be blue.
4-aspect? I thought LU signalling is two-aspect, and what looks like a 4-aspect signal is actually a stop signal combined with a distant for the next signal?
 

Nym

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You can technically have four aspect on the Victoria Line, but it just isn't implemented.
(And of course it was supposed to be the same system on the SSL, but lets not go into that colossal waste of money changing contracts twice)

And the Met Line has always been special in this sense.
 

Banana

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At the end of the day a signalling decision is required, and generally yes it is preferable to keep trains in order where possible. I’d say preserving a decent turnaround time for the Chesham train was probably not a bad call, as 4.5 minutes for an S stock to turn around isn’t much.

Given that the LU train had already got as far as having a clear signal, providing the driver responded immediately (no reason not to) then the Chiltern train would likely only have been checked rather than brought to a complete stand.
I'm not sure the evidence support your second assertion. From the accounts given above the signal went back on the S8 driver before he left the platform. So the Chiltern must have passed the protecting signal and maybe the overlap too. :)
 

Enthusiast

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C: I would assume that the points were set to cross a northbound Chesham from platform 3 to the branch.
Wouldn't it be from platform 1 to the branch? I understand Chalfont to be Platform One - Down; Platform Two - Up; Platform Three - Bay.
 
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Wouldn't it be from platform 1 to the branch? I understand Chalfont to be Platform One - Down; Platform Two - Up; Platform Three - Bay.
I realise it should actually be north/southbound but 1 towards amersham, 2 towards bakers street, 3 bay? (Assuming platforms are numbered 1,2,3 and not 2,1,3)
 

Lewlew

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4-aspect? I thought LU signalling is two-aspect, and what looks like a 4-aspect signal is actually a stop signal combined with a distant for the next signal?
The Met line has sections of 3 and 4 aspect signalling due to the higher speeds
 

bramling

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I'm not sure the evidence support your second assertion. From the accounts given above the signal went back on the S8 driver before he left the platform. So the Chiltern must have passed the protecting signal and maybe the overlap too. :)

Given that the Chesham train was initially on a clear signal (this doesn’t seem to be in doubt) and was just ready to move off when the signal returned to danger, it’s reasonable to assume that had the Chiltern train made a normal controlled approach to the home signal then it may well not have had to stop.

Without having the exact timings to hand it’s hard to say for sure. No doubt it will all come out in the fullness of time.
 

MaxTracks

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To me it seems there are two possibilities that can be speculated - both highly concerning in their own way
either
A) The tripcock/train stop on the Chiltern train failed to activate
or
B) The tripcock activated correctly - meaning that either the train failed to stop in the 900 or so metres from the home signal to the platform, OR was overridden somehow and the driver continued into what could have been a head on collision.
Unless there are any other potential scenarios I haven't considered?
As others have said, I guess we'll await the report.
 

bramling

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To me it seems there are two possibilities that can be speculated - both highly concerning in their own way
either
A) The tripcock/train stop on the Chiltern train failed to activate
or
B) The tripcock activated correctly - meaning that either the train failed to stop in the 900 or so metres from the home signal to the platform, OR was overridden somehow and the driver continued into what could have been a head on collision.
Unless there are any other potential scenarios I haven't considered?
As others have said, I guess we'll await the report.

Ultimately there’s a good handful of possibilities, ranging right through to a serious signalling irregularity. Some of these scenarios are rather more likely than others, however.
 

Taunton

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To me it seems there are two possibilities that can be speculated - both highly concerning in their own way
either
A) The tripcock/train stop on the Chiltern train failed to activate
or
B) The tripcock activated correctly - meaning that either the train failed to stop in the 900 or so metres from the home signal to the platform, OR was overridden somehow and the driver continued into what could have been a head on collision.

Unless there are any other potential scenarios I haven't considered?
Another one is the Chiltern signal was actually green. Which I know will give some apoplexy.
 
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Merle Haggard

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Another one is the Chiltern signal was actually green. Which I know will give some apoplexy.

I recall that after the Clapham collision an expert* talking head on TV that night stated it was definitely driver error (insensitively, as the driver had lost his life) and nothing could possibly go wrong with the signalling.
* who'd achieved 'railway expert' subtitle without ever working in the industry.
 

CyrusWuff

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I would hope that the RAIB investigation will be considering the regulating decision that appears to have been made as a contributing factor. Namely "Why, given what happened at Southall in 1997, was the route set for the Chesham train to cross in front of the Chiltern service in the first place?"

Granted the speeds involved would have been lower, but it doesn't bear thinking about what could have happened had the Met service been crossing over to access the Chesham branch at the time.
 

Surreytraveller

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I would hope that the RAIB investigation will be considering the regulating decision that appears to have been made as a contributing factor. Namely "Why, given what happened at Southall in 1997, was the route set for the Chesham train to cross in front of the Chiltern service in the first place?"

Granted the speeds involved would have been lower, but it doesn't bear thinking about what could have happened had the Met service been crossing over to access the Chesham branch at the time.
Don't be daft. If you make a regulating decision, you'll make even more trains late, causing more conflicts and more regulating decisions to get wrong. Trains should stop at red signals, regardless of the reason for them being red.
 

craigybagel

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I would hope that the RAIB investigation will be considering the regulating decision that appears to have been made as a contributing factor. Namely "Why, given what happened at Southall in 1997, was the route set for the Chesham train to cross in front of the Chiltern service in the first place?"

Granted the speeds involved would have been lower, but it doesn't bear thinking about what could have happened had the Met service been crossing over to access the Chesham branch at the time.
Nonsense. If signallers can't trust their signals to work or drivers to obey them then the whole system would collapse. Even the Southall report, whilst noting that crossing the freight in front of the express did bring about the risk of a collision which sadly came about, absolved the signaler of all blame and said that their assumption that drivers would obey the signals they set is perfectly reasonable.

Since Southall there is even more reason for signallers to assume such moves will be safe, given all junctions with a risk of conflict have some form of protection, mostly through TPWS but in this particular location, train stops. The question here is not why was the LU train signalled the way it was, but why didn't the protection systems keep the two trains safely apart.
 

Surreytraveller

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I do remember quite a few years ago one TOC trying to claim that a SPAD wasn't a driver's fault because the signal they SPADed shouldn't have been red
 

Taunton

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I recall that after the Clapham collision an expert* talking head on TV that night stated it was definitely driver error (insensitively, as the driver had lost his life) and nothing could possibly go wrong with the signalling.
* who'd achieved 'railway expert' subtitle without ever working in the industry.
I actually passed the Waterloo collision a couple of years ago about 30 minutes after it happened. Which was of course a train departing on a green signal.

If you were a driver, coming in on a green signal, hand on the brake handle expecting to stop at the platform, suddenly in the dark hear and feel the train smashing through the mis-set points and so go into emergency braking, where might you possibly end up? Just short of the platform?
 
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Dstock7080

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I would hope that the RAIB investigation will be considering the regulating decision that appears to have been made as a contributing factor. Namely "Why, given what happened at Southall in 1997, was the route set for the Chesham train to cross in front of the Chiltern service in the first place?"
with signals operating correctly the driver would’ve received two cautionary stop signals, before the red.
 

Surreytraveller

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Another one is the Chiltern signal was actually green. Which I know will give some apoplexy.
I'm sure if that was the case, then the railway would have been shut for a lot longer than it was for destructive testing to take place, unless the failed equipment was immediately obvious
 

philthetube

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To answer my question, the Chesham service is timetabled to depart C&L at 21:36.5, well in front of the Chiltern service which passes 6 minutes later.

Why the Chiltern is being delayed for the last service of the day to a single station is beyond me. Anyway, another matter I suppose.

The Chesham branch is single track, if a train is late returning it means that the next service has to remain blocking the main at Chalfont.
Given that the Chesham train was initially on a clear signal (this doesn’t seem to be in doubt) and was just ready to move off when the signal returned to danger, it’s reasonable to assume that had the Chiltern train made a normal controlled approach to the home signal then it may well not have had to stop.

The signal is set far enough back to give plenty of stopping distance before the points. Don't know the distance but must be well over half a mine.
The signal is set far enough back to give plenty of stopping distance before the points. Don't know the distance but must be well over half a mile.
 

philthetube

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Following a Spad the Chiltern driver is required to contact the signaller for permission to proceed, as they cannot make contact by radio this would have to be at a signal phone, so would involve the driver walking back to it so if this happened a minimum delay of five mins would follow, and I would expect much longer.
 

bramling

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Nonsense. If signallers can't trust their signals to work or drivers to obey them then the whole system would collapse. Even the Southall report, whilst noting that crossing the freight in front of the express did bring about the risk of a collision which sadly came about, absolved the signaler of all blame and said that their assumption that drivers would obey the signals they set is perfectly reasonable.

Since Southall there is even more reason for signallers to assume such moves will be safe, given all junctions with a risk of conflict have some form of protection, mostly through TPWS but in this particular location, train stops. The question here is not why was the LU train signalled the way it was, but why didn't the protection systems keep the two trains safely apart.

ISTR that not only this, but furthermore there was also a conclusion that the signaller’s decision at Southall was actually a good regulating decision in the circumstances. It was the most suitable opportunity to get the freight across to where it needed to go.

Southall would very likely not have happened had the AWS been operational or had the train been taken out of service because of that.
 

Mojo

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I would hope that the RAIB investigation will be considering the regulating decision that appears to have been made as a contributing factor. Namely "Why, given what happened at Southall in 1997, was the route set for the Chesham train to cross in front of the Chiltern service in the first place?"

Granted the speeds involved would have been lower, but it doesn't bear thinking about what could have happened had the Met service been crossing over to access the Chesham branch at the time.
There is a very big difference between Southall, and this incident, and that is that, on the Mainline, Overlaps are a fixed length whereas in this case, if a correctly functioning train travelling at the right speed is tripped, then it will come to a stand before the point at which it would come into contact with another train.
 

Surreytraveller

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There is a very big difference between Southall, and this incident, and that is that, on the Mainline, Overlaps are a fixed length whereas in this case, if a correctly functioning train travelling at the right speed is tripped, then it will come to a stand before the point at which it would come into contact with another train.
Which to make it clear to other readers, would be well before the points leading to the Chesham Branch which were run-through
 

357

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Looking at diagrams - there is around 600-700 metres between the signal and the set of trailing points, around 900 metres to where the train stopped.
If the signal was red and the tripcock was activated, I can't see how it would take a 2 car 165 900m to stop from 60mph in emergency.
 
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