Differences between TPWS, ATP and ETCS

[D365]

*used. ATP is no longer in use on the Chilterns.

Is Chiltern stock now fully fitted with Enhanced TPWS?

 

[Snow1964]

TPWS was effectively an upgrade on the old AWS automatic warning system. At the time ATP was hugely expensive and might have only stopped small number of very serious crashes.

AWS was a simple warning that the signal ahead is either green or another aspect. It was placed on approach to a signal, and if not green needed intervention within few seconds or it applied the brakes. The problem was double yellows (and yellows) required acknowledging so it became regular habit and could accidentally happen to a red

TPWS as explained above had the approach replaced by over speed grids (so it only intervened if train was going too fast to stop before danger point). I believe there are 2 settings and freight locos are set at 80% of passenger speed permitted due to different train braking characteristics. From memory mixed traffic locos have a switch that can be set to which time (but not sure about this)

The reason was that in busy areas or at commuter times, many trains do not get green line clear signals, but get regular yellow or double yellow signals (signal after, or 2 after is red), so ok to proceed, but not at unrestricted speeds as signal further ahead is red. A balance between keeping the railway moving and slowing trains approaching red signals.

TPWS therefore has two functions, the over speed (which replaced the simple red or not, but couldn't distinguish the yellow cautions, treating them same as green), and an absolute stop, (the grid adjacent to the signal).

On very slow speed sections (eg approach to terminal platforms) grids are shorter to avoid train getting confused by taking to long to pass over pair of grids and triggering emergency brakes part way along approach to buffers.

 

[Ken H]

So is AWS considered obsolete now. When is an AWS magnet placed at a distant signal today? And is AWS still used at warning boards for speed restrictions?

AWS is a BR stytem for warning the driver about a distant or yellow signal. It uses magnets between the tracks to tell the train equipment of a distant at caution. The driver has to acknowledge a caution aspect.

 

[Railsigns]

Is Chiltern stock now fully fitted with Enhanced TPWS?

Protection was enhanced by fitting extra TPWS loops on the track. Why would the on-train equipment need an upgrade?

 

[najaB]

If there is moisture in the air, a reaction occurs between the different metal contacts in the baseboard and the plugboard of the TPWS module.

That seems like a problem with a (relatively) simple solution...

 

[plugwash]

and if red needed intervention within few seconds or it applied the brakes.

I'm pretty sure AWS only distinguishes between "green" and "not green". Any restrictive aspect has to be acknowledged, as do fixed AWS magnets.

The problem with this was/is that a driver following behind another train can get into a rhythm of acking the double yellow aspects and miss that the next signal is more restrictive than the previous.

So is AWS considered obsolete now.

My understanding is that AWS and TPWS are normally used together, with AWS at every signal on passenger lines and the more complex TPWS at high risk signals.

 

[Ken H]

I'm pretty sure AWS only distinguishes between "green" and "not green". Any restrictive aspect has to be acknowledged, as do fixed AWS magnets.

The problem with this was/is that a driver following behind another train can get into a rhythm of acking the double yellow aspects and miss that the next signal is more restrictive than the previous.


My understanding is that AWS and TPWS are normally used together, with AWS at every signal on passenger lines and the more complex TPWS at high risk signals.

In absolute block areas, they only install AWS at distants. AWS at a stop signal would give the warning/start the automatic brake application too late.
I think in Multi aspect signal areas, every signal that can show yellow gets AWS, and thus you get the repeated AWS warnings.
There was SRAWS (Southern Region AWS, or signal repeating AWS) where the signal aspect was repeated in the cab but that never got widely implemented.

 

[12LDA28C]

Is Chiltern stock now fully fitted with Enhanced TPWS?

I believe so, yes.

Protection was enhanced by fitting extra TPWS loops on the track. Why would the on-train equipment need an upgrade?

Because the latest version of TPWS (sometimes known as TPWS4) has enhanced functionality compared to older versions of TPWS.

So is AWS considered obsolete now. When is an AWS magnet placed at a distant signal today? And is AWS still used at warning boards for speed restrictions?

AWS is a BR stytem for warning the driver about a distant or yellow signal. It uses magnets between the tracks to tell the train equipment of a distant at caution. The driver has to acknowledge a caution aspect.

AWS is certainly not obsolete and provides an audible as well as a visual indication to the driver of signal aspects and speed restrictions. It is used in conjunction with TPWS.

 

[D365]

Protection was enhanced by fitting extra TPWS loops on the track. Why would the on-train equipment need an upgrade?

Because the latest version of TPWS (sometimes known as TPWS4) has enhanced functionality compared to older versions of TPWS.

TPWS4/TPWSfour is a Unipart trademark; Thales calls theirs ”Mk4 TPWS”. All very confusing!

If I’ve understood correctly, ”Enhanced TPWS” is the correct nomenclature for new TPWS with TSS (Train Stop System) functionality as per this RailEngineer feature.

 

[12LDA28C]

TPWS4/TPWSfour is a Unipart trademark; Thales calls theirs ”Mk4 TPWS”. All very confusing!

If I’ve understood correctly, ”Enhanced TPWS” is the correct nomenclature for new TPWS with TSS (Train Stop System) functionality as per this RailEngineer feature.

Quite possibly so, but most people I know who are directly involved with train operation (i.e. not railway engineers) just refer to it as TPWS4 which seems to align with both trademarks you describe.

 

[swt_passenger]

TPWS4/TPWSfour is a Unipart trademark; Thales calls theirs ”Mk4 TPWS”. All very confusing!

If I’ve understood correctly, ”Enhanced TPWS” is the correct nomenclature for new TPWS with TSS (Train Stop System) functionality as per this RailEngineer feature.

What I’m getting from that article is that Mk4 TPWS aka TPWS4 adds the three separate indications in the cab telling the driver the cause of a brake demand, but doesn’t actually change the way TPWS works. In other words it is a user interface improvement, and implies that stock can be retrofitted with a Mk 4 onboard system without doing anything to the trackside equipment?

“On-train TPWS equipment would be upgraded to the most recent design standard, known as Mark 4 TPWS, which benefits from design changes to improve its effectiveness. Compared to earlier TPWS control panels, Mark 4 features three separate indicators to show the cause of a brake demand – SPAD, Overspeed or AWS. It also adds a covered ‘Brake Release’ button, to involve the driver in the brake release process.”

 

[12LDA28C]

What I’m getting from that article is that Mk4 TPWS aka TPWS4 adds the three separate indications in the cab telling the driver the cause of a brake demand, but doesn’t actually change the way TPWS works. In other words it is a user interface improvement, and implies that stock can be retrofitted with a Mk 4 onboard system without doing anything to the trackside equipment?

“On-train TPWS equipment would be upgraded to the most recent design standard, known as Mark 4 TPWS, which benefits from design changes to improve its effectiveness. Compared to earlier TPWS control panels, Mark 4 features three separate indicators to show the cause of a brake demand – SPAD, Overspeed or AWS. It also adds a covered ‘Brake Release’ button, to involve the driver in the brake release process.”

This is correct. The improvement of TPWS4 over previous versions is in functionality for the driver with no necessary upgrade to track-based equipment.

 

[Annetts key]

TPWS was effectively an upgrade on the old AWS automatic warning system. At the time ATP was hugely expensive and might have only stopped small number of very serious crashes.

TPWS is an additional safety system that is selectivity fitted. It is normally only fitted where there is a potential conflict point after (beyond) the signal (and level crossings normally don't count). Unless a design specifies otherwise. Note that here, the phrase conflict point means a junction or points that allow another movement across, converging with the line that the signal is protecting.

I only speak about GWML ATP, as that is the type that I am familiar with.

IMHO, the costs that are used to justify not proceeding with ATP are misleading. They include fitting every single main aspect or semaphore signal in the country as part of a large scheme that would therefore require specific large funding and which would most likely bring most, if not all other signalling work to a halt. In addition, every single loco, EMU, DMU etc, would need to be taken out of service to be fitted with ATP.

Also keep in mind that at the time, it was not mandatory for signals to be fitted with AWS. And indeed, there were both freight and passenger lines that were not fitted with AWS. Yes, these were mostly branch lines. So with a national ATP fitment, signals that did not even have AWS would have been required to have ATP fitted.

In the real world, a far more sensible approach would have been to add a requirement to all new trains to be ATP fitted during manufacture. In the long term, this would have saved money. Looking back, how many new trains have been introduced since 1993?

Then to fit the high speed, main lines, or busy lines first. To fit the trains that travelled on these lines first. And to roll out the above as a rolling programme. Leaving all other lines later to when the existing signalling was due to be upgraded, renewed, replaced due to other reasons.

Also, it needs to be realised that fitting GWML ATP to a multi-aspect colour light signal does not require ANY alterations to the interlocking whatsoever unless said signal has a route indicator (any type). There are no alterations to the circuitry in the signal box/panel.

For signals that do have a route indicator (any type), the alteration is very minor.

Hence, with the vast majority of signals, fitting GWML ATP is considerably cheaper than fitting the very same signal with TPWS (all signals fitted with TPWS require alterations to the interlocking, and a lot will also require alterations to the circuitry in the signal box/panel).

Hence when the costs of ATP (as a national project) are compared with TPWS (where only a fraction of signals are fitted) is IMHO very misleading.

No one will ever know what the outcome of fitting ATP on at least the high speed, main lines, or busy lines would have been. But it would have most certainly have saved lives, reduced the number of injuries and reduced the number of over-speeding incidents.

You may call AWS old, but it's not the oldest system with similar features. GWR had its ATC (Automatic Train Control) system for example, which had the same functionality.

AWS is still a current standard and for a lot of signals, is the only 'safety system'.

AWS was a simple warning that the signal ahead is either red or another aspect. It was placed on approach to a signal, and if red needed intervention within few seconds or it applied the brakes.

AWS is a warning system that alerts the driver when a signal is not showing a green aspect (or equivalent). If the driver does not acknowledge the warning, after a short delay, the train brakes will automatically be applied.

To be clear, if the signal is not lit (black/no light), is red, single yellow, double yellow, or at caution, then a horn (or equivalent noise) sounds in the cab.

Only if the signal is showing green (or off) will the bell (or equivalent noise) sound in the cab.

AWS also has the advantage that the train will still detect each installation even if there is no electrical power to any of the signalling system. As the permanent magnets used work without needing a power source (the rest of the system does need electrical power).

AWS and other similar systems (such as GWR ATC) have saved many, many lives. If this has been proposed to be brought in as one large project like costed for ATP, AWS would never have happened either....

TPWS as explained above had the approach replaced by over speed grids (so it only intervened if train was going too fast to stop before danger point). I believe there are 2 settings and freight locos are set at 80% of passenger speed permitted due to different train braking characteristics. From memory mixed traffic locos have a switch that can be set to which time (but not sure about this)

The reason was that in busy areas or at commuter times, many trains do not get green line clear signals, but get regular yellow or double yellow signals (signal after, or 2 after is red), so ok to proceed, but not at unrestricted speeds as signal further ahead is red. A balance between keeping the railway moving and slowing trains approaching red signals.

TPWS therefore has two functions, the over speed (which replaced the simple red or not, but couldn't distinguish the yellow cautions, treating them same as green), and an absolute stop, (the grid adjacent to the signal).

For the TPWS equipment used for the track based / line side part of the system, all the loops ("grids", "toast racks") are exactly the same. Apart from where the line speed is low, in which case shorter versions are available (typically seen in platform lines). The differences are in the position, the spacing between pairs of loops and in the frequency transmitted by each loop.

Each TPWS installation is built up using different combinations of standard parts.

A typical installation is a signal with an OSS (Over Speed System) and a TSS (Train Stop System).

The OSS has two loops, an "arming" and a "trigger" some distance on approach to the signal. The distance between these two loops sets the trigger speed. If a train passes at or above the set speed, the train brakes will be applied. To prevent trains being affected when the signal is showing a proceed aspect, these loops are powered down.

The distance away from the signal is related to where the first potential conflict point after (beyond) the signal is and the line speed

TPWS provided for a speed restriction is effectively an OSS that is always powered.

TPWS provided for movements towards buffer stops is also an OSS that is always powered.

TPWS+ also known as OSS+ is an additional OSS installation used at 'high-risk' locations, increasing the effectiveness to 100MPH. OSS+ is mounted at a greater distance away from the potential conflict point.

A TSS is always provided at a signal that is required to have TPWS. As previously described, a pair of loops is mounted close together in line in one arrangement such that from a train, it looks like one long loop. A powered TSS will always trigger the brakes if a train passes over it. The TSS loops are powered down when the signal shows a proceed aspect.

Because the speed and braking capacity and other characteristics vary between different train types, and TPWS is not a computer based system, the duration of the timer used in the train equipment is different for passenger class trains and freight trains. Thus, for a freight train, a OSS set speed is lower than for a passenger train at the same OSS.

TPWS does not solve the problem of trains encountering multiple signals showing caution (yellow or double yellow) aspects. As many of these signals will not have a potential point of collision beyond the signal, hence there is normally no requirement for TPWS to be fitted...
The problem with drivers encountering multiple consecutive signals showing caution, is that with them repeatedly acknowledging the warning from the AWS, this may become routine. So when the next signal is showing a red, they get the same horn, they automatically acknowledge the warning from the AWS, then if they are not paying attention, they may find that they cannot stop their train before passing the signal at danger. ATP does solve this issue.

That seems like a problem with a (relatively) simple solution...

The plug in modules had a specification change. Now the plugboard contacts must be gold plated. However, there is no requirement for the baseplate contacts to be gold plated. So although it reduces the problem (well, delays the problem), it does not solve it. As you still have two different metals... The powers that be presumably know about it. But someone must have done a cost benefit analyst and decided not to make any further changes.

 

[najaB]

But it would have most certainly have saved lives, reduced the number of injuries and reduced the number of over-speeding incidents.

What fatal accident(s) would ATP have prevented that TPWS didn't?

 

[Annetts key]

What fatal accident(s) would ATP have prevented that TPWS didn't?

Those that occurred before TPWS was fitted for a start.

 

[Efini92]

What fatal accident(s) would ATP have prevented that TPWS didn't?

Ladbroke grove and southall would’ve been prevented. Granted they both occurred before tpws was fitted.

Hi all

I'm trying to grasp what the differences is between TPWS, ATP and ETCS. I think I understand the benefits of ETCS from the perspective of efficiency and capacity.

But what is the difference between these systems from a safety perspective. I read that TPWS provides 70% of the protection of ATP for SPADS. What accounts for this reduction in safety? What can ATP do that TPWS cannot?

I read that TPWS doesn't stop a train before a red light but just after it but before and critical junctions etc. If that's true, why does it work that way? Is it that which accounts for the fact that it's not quite as safe at ATP?

How do they both compare to ETCS from a safety perspective?

Does each system have any other safety benefits other than preventing running red lights?

I understand that all the systems are effective, but just trying to understand the differences.

I have tried searching online but there doesn't seem to be a simple explanation.

Thanks!

In a nutshell ATP prevents spads, TPWS mitigates the risk of a spad by stopping the train before it reaches the point of conflict.

 

[D365]

The plug in modules had a specification change. Now the plugboard contacts must be gold plated. However, there is no requirement for the baseplate contacts to be gold plated. So although it reduces the problem (well, delays the problem), it does not solve it. As you still have two different metals... The powers that be presumably know about it. But someone must have done a cost benefit analyst and decided not to make any further changes.

Is this relevant to the FPGA (Field Programmable Gate Array) architectures of ”Enhanced TPWS” control units?

 

[Ken H]

@Annetts key. One advantage on AWS is the unpowered fit so every train gets the caution. So a simple installation for fixed distants and speed restrictions. And a temporary fit for temporary speed restrictions.

Also worth a mention re AWS is the visual display. After a 'hooter' the driver sees a yellow and black sunflower to remind them they have passed a signal at caution.

 

[Annetts key]

Is this relevant to the FPGA (Field Programmable Gate Array) architectures of ”Enhanced TPWS” control units?

I don't know. I was talking about the modules in the line side location cases / cupboards that Network Rail are responsible for. As used for all TPWS installations.
The modules look like this:

 

[D365]

I don't know. I was talking about the modules in the line side location cases / cupboards that Network Rail are responsible for. As used for all TPWS installations.
The modules look like this:View attachment 153903

Apologies, I’m talking at cross-purposes. I should really remember better from my BST (Basic Signalling Training)

 

[Annetts key]

@Annetts key. One advantage on AWS is the unpowered fit so every train gets the caution. So a simple installation for fixed distants and speed restrictions. And a temporary fit for temporary speed restrictions.

Absolutely, yes.

A fixed AWS magnet to create a horn can be installed in around one, maybe two hours (assuming the sleeper spacing is okay and the sleepers are suitable).

A temporary AWS magnet (for a TSR - temporary speed restriction or ESR - emergency speed restriction) can be fitted by the P.Way in less than ten minutes. That's per magnet BTW.

GWML ATP (per signal) can be adjusted for an unsupervised ESR in around 10 to 15mins (per signal) or for a TSR if pre-programmed parameter plugs ("chips") are available. Of course, the time it takes to get to the signal from the access point is another thing. Especially if multiple signals need their ATP adjusting (typically it's three signals per line).

Now compare to TPWS. If you want to provide TPWS (OSS) to 'enforce' a TSR or a ESR, then a new installation is needed. I've not seen this happen in practice on my area. I imagine it always falls into the too difficult box.

 

[Nym]

TPWS4/TPWSfour is a Unipart trademark; Thales calls theirs ”Mk4 TPWS”. All very confusing!

If I’ve understood correctly, ”Enhanced TPWS” is the correct nomenclature for new TPWS with TSS (Train Stop System) functionality as per this RailEngineer feature.

Thanks for posting the "Information Post" (a.k.a.) rant to save me from doing it.

One could go back even further in that what most call TPWS is actually "TPS". The RSSB doesn't exactly help themselves by talking very much cross purposes for what TPWS actually is, and the requirements for nomenclature of cab and train equipment.

eg.

TPWS Temporary Isolation Switch
TPWS Full Isolation Switch
AWS Isolation Switch.

The fun thing being.

If you take an AWS Isolation switch that's pre "Enhanced TPWS" then that is actually now a TPWS Full Isolation Switch, and the new AWS Isolation Switch doesn't do the same as the old AWS Isolation Switch. So a driver used to an AWS Isolation Switch, will have a new AWS Isolation switch that does something completely different to the old AWS isolation switch, which the chances are, will result in incorrect applications of isolation. And all of this could be solved by the RSSB making a better choice when it comes to the naming of HMI components.

And before anyone thinking this is a manufacturers' issue, this is as defined by the RSSB!

 

[craigybagel]

What I’m getting from that article is that Mk4 TPWS aka TPWS4 adds the three separate indications in the cab telling the driver the cause of a brake demand, but doesn’t actually change the way TPWS works. In other words it is a user interface improvement, and implies that stock can be retrofitted with a Mk 4 onboard system without doing anything to the trackside equipment?

“On-train TPWS equipment would be upgraded to the most recent design standard, known as Mark 4 TPWS, which benefits from design changes to improve its effectiveness. Compared to earlier TPWS control panels, Mark 4 features three separate indicators to show the cause of a brake demand – SPAD, Overspeed or AWS. It also adds a covered ‘Brake Release’ button, to involve the driver in the brake release process.”

This is correct. The improvement of TPWS4 over previous versions is in functionality for the driver with no necessary upgrade to track-based equipment.

As far as I'm aware, the only safety upgrade in TPWS4 is that It also removes the ability to bypass the 60 second timer before a brake activation can be reset by keying in and out of the cab.

 

[Nym]

As far as I'm aware, the only safety upgrade in TPWS4 is that It also removes the ability to bypass the 60 second timer before a brake activation can be reset by keying in and out of the cab.

Instead you can just bypass that by using Aux Trip / Aux Set, tripping and resetting the MCB or isolation and de-isolation of the TPWS equipment. I doubt, although there might be, a Real Time Clock provided by the TPWS "FPGA" units.

 

[craigybagel]

Instead you can just bypass that by using Aux Trip / Aux Set, tripping and resetting the MCB or isolation and de-isolation of the TPWS equipment. I doubt, although there might be, a Real Time Clock provided by the TPWS "FPGA" units.

True, but in a lot of stock the first option will talk longer than 1 minute, and the latter options may well involve breaking seals. It's still making it a lot harder for a driver to attempt to cover up a misdemeanor.

 

[najaB]

What fatal accident(s) would ATP have prevented that TPWS didn't?

Those that occurred before TPWS was fitted for a start.

I don't think it's really fair to say that one system that wasn't fitted would "almost certainly" have saved lives as compared to another system that wasn't fitted.

 

[craigybagel]

I don't think it's really fair to say that one system that wasn't fitted would "almost certainly" have saved lives as compared to another system that wasn't fitted.

Especially as since network wide TPWS was installed after Ladbroke Grove, the number of fatalities caused by SPADs or overspeeds is precisely 0. We were averaging a fatal accident caused by one of those every 2 to 3 years.

 

[D365]

Instead you can just bypass that by using Aux Trip / Aux Set, tripping and resetting the MCB or isolation and de-isolation of the TPWS equipment. I doubt, although there might be, a Real Time Clock provided by the TPWS "FPGA" units.

True, but in a lot of stock the first option will talk longer than 1 minute, and the latter options may well involve breaking seals. It's still making it a lot harder for a driver to attempt to cover up a misdemeanor.

Train operators will rely on the JRU (data recorder) to catch drivers who attempt to ’cheat’ the penalty time.

 

[12LDA28C]

In a nutshell ATP prevents spads, TPWS mitigates the risk of a spad by stopping the train before it reaches the point of conflict.

It's still entirely possible to have a SPaD with ATP fitted and working.

 

[Efini92]

It's still entirely possible to have a SPaD with ATP fitted and working.

Is it? I thought that was the whole point of the system.