PSR

[fil]

Hi,
I need a little guidance on permanent speed resrictions.
When they are used, how they are classified ect.
If they are permanent why are they not just part of the signalling scheme?
Are diverging junctions (@>10mph difference with mar/may) classed as Psr?
Is every psr indicated to the driver?
Abit of an overview would be great, or direction on where to find info.

Thanks ppl.

[PJW]

Hi,
I need a little guidance on permanent speed resrictions.
When they are used, how they are classified etc.

First a little piece of historical context that should aid understanding.

Railways in the UK still owe much to their historic inheritance- the bad as well as the good. Many of the lines that we now think of main trunk lines were not built as such; the railways were not initially conceived as a network but just relatively local, and isolated, lines from A to B, C to D etc and only after time was a line built from B to C to link them and these lines could have been constructed by separate companies. Not always the case; the GWR from London to Bristol is one obvious exception, but then the Bristol to Exeter was built but approached nearly at right angles- hence thevery curved platforms which are a feature of Bristol Temple Meads station today.

Similarly local interests often dominated and therefore a line could be planned to go out of its way in order to serve the needs of a particular major shareholder or indeed conversely avoid the land of a particularly influential landowner who didn't want the noise and disruption of a railway near their property. Many lines were built relatively "on the cheap" so may contour around the land, following the line of rivers. In the early days a speed of 30mph was considered pretty fast, so quite sharp curves weren't a problem; why would you align the railway for >100mph running in that environment?

Hence our railways are often curvaceous over sections, or indeed two initially separate railways joined together, and this could well mean a relatively sharp curve on what would otherwise be reasonably straight railway. Various straighting exercises have taken place over the intervening decades, but the UK is a pretty crowded island and therefore the amount that this can be done is limited.

Some other country's railways were much more carefully planned as an entity by government etc; you need to understand that in general the UK railways "just grew" relatively haphazardly rather than being strongly centrally planned- a penalty of being the pioneers. This also led to quite a few competing lines that were arguably always superfluous to the actual transport need and certainly with the decline in utilisation post World War 2 there were many line closures. This led to another round of needing to connect "this bit of line A" with "that bit of line B", so a few more sharp curves at junctions were created.

So to get to the point; the speed profile along any line is generally complex with some portions capable of much lower speed than the highest speed section of the route. Until relatively recently a nominal "line speed" was defined for the long length of line (i.e. the highest speed limit anywhere along it) and anywhere along the length where that speed would not be safe / comfortable / permissible without excessive wear & tear on weak structures etc. was defined to have a "Permanent Speed Restriction". The driver had to have "route knowledge" so that they knew what the speed limit was at any location along the line; purely to remind them the commencement and end of the restriction were marked by signs consisting of "cut-out" large numbers (denoting speed limit in mph). However after a number of over-speeding accidents as a result of the driver failing to remember in time, warning boards were also provided at the braking distance needed that application of the brakes at that site would ensure compliance with the forthcoming PSR.

Although many of us still refer to "linespeed" on Network Rail, this is officially an out-dated term. Nowadays we regard every route of having a "speed profile" and we provide "continuous route signage" where every change of speed is signed and there are AWI [Advance Warning Indicators] at braking distance prior. This includes signage with associated sideways arrows to show the speed over the curved path through turnouts.

Actually it is rather more complicated than that because we often have "differential speed signage" because of different train characteristics. For example:

1. a relatively light weigh multiple-unit train may well be able to go faster around a curve or over a viaduct than a heavy locomotive hauled train would be permitted to travel.
   
2. similarly the freight trains probably have worse braking than a passenger train and to maximise speed and headway for the passenger service then the signalling would be optimised for that traffic but as a consequence could then be "under-braked for a freight train moving at the same top speed.; in this case dual signage would be provided to fix a lower speed limit for the freight that has been calculated to make sure that its braking from that speed would fit within the braking distance for the higher speed trains with better brakes.
   

It gets even more complicated as on some routes there is also EPS [Enhanced Permissible Speed] on which certain specific trains are allowed to travel faster than the normal ones, generally because these are arranged to tilt when going around curves and thus the higher speed can be utilised without giving excessive passenger discomfort.

Don't even get me started regarding the need for speed signage on NR's only ETCS fitted route.........

If they are permanent why are they not just part of the signalling scheme?

So the PSR / speed profile very much ARE part of the signalling scheme; they have an important effect on what is SAFE re braking and obviously affect CAPACITY. Speed signage is shown on the signalling plan and checked by the signalling testers (even if physically erected by the P'Way in many instances). You haven't got the time in the IRSE Exam to add them all and they certainly add a lot of clutter; the IRSE examiners don't seem to feel they are essential (partly I think that the UK mainline approach has changed significantly "since they were a lad"). I do recommend those claiming to follow modern (NR) practices in module 2 to add a general note re the fact that "continuous speed signage would be provided but not shown" in just the way as TPWS / AWS etc aren't really key items to show but conversely shouldn't be totally disregarded without any comment at all.

Are diverging junctions (@>10mph difference with mar/may) classed as Psr?

I'd have to check the official definitions (and I wouldn't be confident that all sources would be 100% consistent!). In "old-speak" I certainly would not call them PSRs (although of course the driver would need to know them and the speed through junctions was always shown on the diagram within the "Sectional Appendix" which is also the document which defined the "linespeed" and those PSRs en route).

In "new-speak" with "continuous speed signage" then they are shown; the same speed signs are used but qualified by the addition of an arrow to left or right as applicable (but often the smaller signs are utilised as not needing to be read at maximum speed). However as I have said above, strictly the term PSR is not applicable in this terminology.

Note that the speeds are shown regardless; if they are different at all the values are shown (but you are right that the signalling effectively treats speeds that only differ by 10mph or less as effectively close enough to be the same as far as approach release is concerned). Obviously if the speeds through either lie of the points are equal they are not shown if the same as the speed on the preceeding stretch of track, however they are both shown if for example there is a Y point with each divergence 70mph with the approach speed to it being 80mph.

Is every psr indicated to the driver?

See above; the simple answer is YES (but whether just as a PSR commencement where the permissible speed does a step-change on the straight through route, or as an element within "continuous speed signage" does vary between routes)

A bit of an overview would be great, or direction on where to find info.
Thanks ppl.

Probably more than you really wanted; it sounded quite an innoculous question, but actually there is quite a lot of complexity arising from it.

Basically it all arises from efforts to squeeze as much performance as we can from the railway that we have got, when not able to justify the large costs and the other disadvantages (such as destroying someone's property in the neighbourhood in order to be able to straighten the railway) of making major structural improvements to the alignment. Which is why I started this description re the historical legacy that is at the heart of all this....

Of course there are currently attempts to build a high speed line from London to Birmingham that wouldn't suffer from such a legacy. Coming up against a fair bit of opposition and I can understand why; avoiding as much as possible any built-up area means taking it through what is an area of outstanding natural beauty and therefore destruction of what is at present a relatively quiet and pleasant piece of countryside that many from London and elsewhere value for what it is now. I myself am very torn; if it came to a public referendum I wouldn't know which way to vote......

[Jerry1237]

As something else to think about, what is the difference between an ESR and a TSR (E == emergency, T == temporary). Also, which is more [sic] risky and why?

[PJW]

As something else to think about, what is the difference between an ESR and a TSR (E == emergency, T == temporary).

The TSR should not come as a surprise to the driver; it will already have been published via the Weekly Operating Notice or at least posted in the Late Notice case at the signing-on points. Generally they are pre-planned, perhaps following some P'Way relaying with the line reopened but not yet at its full speed. Sometimes as a response to an assessment of the gradually deteriorating state of the track, embankment, structure etc.

The ESR is an ugent response to something pretty serious that has just happened / been noticed; tend to be of short physical length but of extremely low speed, perhaps due to a specific rail defect. Because they will be otherwiise unexpected, drivers should wherever possible be warned prior and a black / yellow chevroned marker board with flashing high intensity flashing lights are placed on the approach.

Due to therir nature ESRs are generally targetted for extremely rapid rectification. Some TSRs were actually in practice long-term, particularly those imposed due to infrastructure deterioration which was limited and the effect on traffic insufficient to justify big expenditure to resolve. There were a huge number of TSRs imposed after the Hatfield accident and the scale of the remedial work needed nationally meant that resolving them was a slow process. The rules now are that TSRs should not be in force for more than a year; the effect of this is that some long term TSRs have been converted into PSRs- this of course means incorporation into the Sectional Appendix, modifying the speed signage of the route etc.

Also, which is more [sic] risky and why?

I think this is a bit of a Mod1 question for a student to attempt!

[fil]

Hi,
I need a little guidance on permanent speed resrictions.
When they are used, how they are classified etc.

First a little piece of historical context that should aid understanding.

Railways in the UK still owe much to their historic inheritance- the bad as well as the good. Many of the lines that we now think of main trunk lines were not built as such; the railways were not initially conceived as a network but just relatively local, and isolated, lines from A to B, C to D etc and only after time was a line built from B to C to link them and these lines could have been constructed by separate companies. Not always the case; the GWR from London to Bristol is one obvious exception, but then the Bristol to Exeter was built but approached nearly at right angles- hence thevery curved platforms which are a feature of Bristol Temple Meads station today.

Similarly local interests often dominated and therefore a line could be planned to go out of its way in order to serve the needs of a particular major shareholder or indeed conversely avoid the land of a particularly influential landowner who didn't want the noise and disruption of a railway near their property. Many lines were built relatively "on the cheap" so may contour around the land, following the line of rivers. In the early days a speed of 30mph was considered pretty fast, so quite sharp curves weren't a problem; why would you align the railway for >100mph running in that environment?

Hence our railways are often curvaceous over sections, or indeed two initially separate railways joined together, and this could well mean a relatively sharp curve on what would otherwise be reasonably straight railway. Various straighting exercises have taken place over the intervening decades, but the UK is a pretty crowded island and therefore the amount that this can be done is limited.

Some other country's railways were much more carefully planned as an entity by government etc; you need to understand that in general the UK railways "just grew" relatively haphazardly rather than being strongly centrally planned- a penalty of being the pioneers. This also led to quite a few competing lines that were arguably always superfluous to the actual transport need and certainly with the decline in utilisation post World War 2 there were many line closures. This led to another round of needing to connect "this bit of line A" with "that bit of line B", so a few more sharp curves at junctions were created.

So to get to the point; the speed profile along any line is generally complex with some portions capable of much lower speed than the highest speed section of the route. Until relatively recently a nominal "line speed" was defined for the long length of line (i.e. the highest speed limit anywhere along it) and anywhere along the length where that speed would not be safe / comfortable / permissible without excessive wear & tear on weak structures etc. was defined to have a "Permanent Speed Restriction". The driver had to have "route knowledge" so that they knew what the speed limit was at any location along the line; purely to remind them the commencement and end of the restriction were marked by signs consisting of "cut-out" large numbers (denoting speed limit in mph). However after a number of over-speeding accidents as a result of the driver failing to remember in time, warning boards were also provided at the braking distance needed that application of the brakes at that site would ensure compliance with the forthcoming PSR.

Although many of us still refer to "linespeed" on Network Rail, this is officially an out-dated term. Nowadays we regard every route of having a "speed profile" and we provide "continuous route signage" where every change of speed is signed and there are AWI [Advance Warning Indicators] at braking distance prior. This includes signage with associated sideways arrows to show the speed over the curved path through turnouts.

Actually it is rather more complicated than that because we often have "differential speed signage" because of different train characteristics. For example:

1. a relatively light weigh multiple-unit train may well be able to go faster around a curve or over a viaduct than a heavy locomotive hauled train would be permitted to travel.
   
2. similarly the freight trains probably have worse braking than a passenger train and to maximise speed and headway for the passenger service then the signalling would be optimised for that traffic but as a consequence could then be "under-braked for a freight train moving at the same top speed.; in this case dual signage would be provided to fix a lower speed limit for the freight that has been calculated to make sure that its braking from that speed would fit within the braking distance for the higher speed trains with better brakes.
   

It gets even more complicated as on some routes there is also EPS [Enhanced Permissible Speed] on which certain specific trains are allowed to travel faster than the normal ones, generally because these are arranged to tilt when going around curves and thus the higher speed can be utilised without giving excessive passenger discomfort.

Don't even get me started regarding the need for speed signage on NR's only ETCS fitted route.........

If they are permanent why are they not just part of the signalling scheme?

So the PSR / speed profile very much ARE part of the signalling scheme; they have an important effect on what is SAFE re braking and obviously affect CAPACITY. Speed signage is shown on the signalling plan and checked by the signalling testers (even if physically erected by the P'Way in many instances). You haven't got the time in the IRSE Exam to add them all and they certainly add a lot of clutter; the IRSE examiners don't seem to feel they are essential (partly I think that the UK mainline approach has changed significantly "since they were a lad"). I do recommend those claiming to follow modern (NR) practices in module 2 to add a general note re the fact that "continuous speed signage would be provided but not shown" in just the way as TPWS / AWS etc aren't really key items to show but conversely shouldn't be totally disregarded without any comment at all.

Are diverging junctions (@>10mph difference with mar/may) classed as Psr?

I'd have to check the official definitions (and I wouldn't be confident that all sources would be 100% consistent!). In "old-speak" I certainly would not call them PSRs (although of course the driver would need to know them and the speed through junctions was always shown on the diagram within the "Sectional Appendix" which is also the document which defined the "linespeed" and those PSRs en route).

In "new-speak" with "continuous speed signage" then they are shown; the same speed signs are used but qualified by the addition of an arrow to left or right as applicable (but often the smaller signs are utilised as not needing to be read at maximum speed). However as I have said above, strictly the term PSR is not applicable in this terminology.

Note that the speeds are shown regardless; if they are different at all the values are shown (but you are right that the signalling effectively treats speeds that only differ by 10mph or less as effectively close enough to be the same as far as approach release is concerned). Obviously if the speeds through either lie of the points are equal they are not shown if the same as the speed on the preceeding stretch of track, however they are both shown if for example there is a Y point with each divergence 70mph with the approach speed to it being 80mph.

Is every psr indicated to the driver?

See above; the simple answer is YES (but whether just as a PSR commencement where the permissible speed does a step-change on the straight through route, or as an element within "continuous speed signage" does vary between routes)

A bit of an overview would be great, or direction on where to find info.
Thanks ppl.

Probably more than you really wanted; it sounded quite an innoculous question, but actually there is quite a lot of complexity arising from it.

Basically it all arises from efforts to squeeze as much performance as we can from the railway that we have got, when not able to justify the large costs and the other disadvantages (such as destroying someone's property in the neighbourhood in order to be able to straighten the railway) of making major structural improvements to the alignment. Which is why I started this description re the historical legacy that is at the heart of all this....

Of course there are currently attempts to build a high speed line from London to Birmingham that wouldn't suffer from such a legacy. Coming up against a fair bit of opposition and I can understand why; avoiding as much as possible any built-up area means taking it through what is an area of outstanding natural beauty and therefore destruction of what is at present a relatively quiet and pleasant piece of countryside that many from London and elsewhere value for what it is now. I myself am very torn; if it came to a public referendum I wouldn't know which way to vote......

OK, so the image i had in my mind of a psr was basically a TSR which was a permanent fixture, i.e it had count down markers and commencment board using light weight structured signs like a tsr.

the way i understand the psr now is: any change to the speed profile from the maximum speed.
i didnt realise that the lower speeds (speed boards) were classed as a psr.
this is why i was confused, i couldn't faverm why all of the above you mention was not implemented into a scheme at the design stage and why we continued to use tsr type psr.

although, i do not recall ever seeing a PSR awi while out on track or on a scheme plan.

A very nice explaination PJW, i really appreciate the help.
fil

[PJW]

OK, so the image i had in my mind of a psr was basically a TSR which was a permanent fixture, i.e it had count down markers and commencment board using light weight structured signs like a tsr.

the way i understand the psr now is: any change to the speed profile from the maximum speed.
i didnt realise that the lower speeds (speed boards) were classed as a psr.
this is why i was confused, i couldn't fathom why all of the above you mention was not implemented into a scheme at the design stage and why we continued to use tsr type psr.

although, i do not recall ever seeing a PSR awi while out on track or on a scheme plan.

fil

I suspect you have but didn't recognise for what they were.
See attached a couple of photos- one of a pair of AWI for a differential speed restriction and the other the commencement of a PSR and an AWI warning for a forthcoming even more severe PSR.

[reuben]

And some more comments from me....

All types of speed restriction / speed information have tended to be rather neglected in the mind of UK signal engineers because, for the most part, it just sits there and does it's job. If it's not got any lights or arms to operate or prove, signal engineers tend to get switched off by it. (or is that switched ON ?). Hence the general assumption in Module 2 is that speed signage is not of interest, and in fact most signal engineers tend to mentally skip over the symbols on the plan, even if they're there in the first place. Furthermore, maintenance of lineside signage has often been delegated to other departments, so even more diminishing its apparent importance.

Some definitions - personal opinions rather than word-perfect from the standards. assume Uk mainline current practice.

Permitted speed.

Every point on the railway has a maximum permitted speed. This may originally have been imposed for safety, asset life or passenger comfort reasons. However, if a (non safety) reason has dictated a lower permitted speed, this also becomes the safety limit, because the track engineer is not obliged to maintain track for safe use at a higher non-permitted speed. The permitted speed may well have differential ( freight/ passenger/ whizzy passenger with better brakes ) values at any place. The permitted speed may be different for opposite directions on the same track. Current practice of continuous speed signage informs the driver of every change in permitted speed.

A "permanent speed restriction" is therefore any place at which the permitted speed falls.

Braking distances (aka signal spacing)

Wherever there is a stop signal, there must be a first caution ( yellow, double yellow , semaphore distant signal etc) at full braking distance on the approach to it, based on the permitted speed at the post of that cautionary signal, and the worst possible braking capabilities of the trains allowed to approach it. This is post to post, so sighting of the distant and overlap at the stop signal are both extra allowances. The only exception is if you can prove that the attainable speed for a "best case" accelerating train can never reach the permitted speed (eg leaving a terminus station). The same philosophy applies to warning of speed restrictions.

Route Knowledge

The signalling arrangement therefore assumes that each train is already travelling within the permitted speed envelope when approaching every caution signal. "Route knowledge" actually comprises:
What's in the driver's head - he was trained and assessed on the line's speed profile before being passed as competent to drive it.
Lineside signs, for when the driver forgets
The sectional appendix, for when the signs fall over!

There's therefore triple diversity in the "route knowledge"

"Temporary Speed Restrictions" are advised to the driver by the weekly operating notice and lineside signage - hence dimple dual diversity here, mitigated by time limitations.

"Emergency Speed restrictions", regardless of the technical origin, are defined to be where the weekly operating notice communication is inadequate or missing. To provide dual diversity, these have a double set of lineside signage, the first set using a visually arresting flashing beacon.

Line Speed

As Peter says, an often used but strictly superseded term. In general, the costs of building, running and maintaining a railway system (trains and infrastructure together) are proportional to the square (or some similar higher power) of the speed. Therefore if you double the speed, you quadruple the costs. It therefore follows that if you're building / upgrading a railway from A to B, the optimum speed profile is constant speed. The greater the number of passengers and the greater the perceived importance of the route, the more money there will be to reduce journey times by raising this constant speed profile along the entire route. So the line speed is a business led decision: we're not prepared to spend more cash to go any faster on this line. However, there are always localities where the engineering constrains the ability to reach this aspirational speed - permanent speed restrictions.

In the old days, before continous speed signage , the driver only needed to know the Line Speed and the local deviations from that (Permanent Speed Restrictions), making the route knowledge less onerous.

If you leaf through a sectional appendix you will see this: between two major centres A and B, the permitted speed mostly sits at a constant ceiling value (the Line speed) , with occasional local reductions due to infrastructure limitations (PSRs). The same railway may then carry on from B to C, but it may well have a different ceiling speed: with different numbers of passengers the business case mathematics will give a different result even though it's the same railway.

The general UK practice is not to use signal aspects to warn train drivers of permanent (or temporary) speed restrictions - the reliance being placed on "Route Knowledge".

However, as a mitigation for drivers occasionally forgetting severe permanent speed restrictions (Google for Morpeth train derailments!), it has become the practice to provide permanent AWS magnets (as for a yellow signal) to draw the driver's attention to the advanced warning indicator sign, and a TPWS installation to invoke a brake application in the event of a major misjudgement.

Finally we come to junction signalling - a place where one signal can authorise a choice of routes. Usually the permitted speeds will be different over the two routes. The key thing is that the driver's "route knowledge" for the two routes is assumed to be perfect, but the driver may wrongly anticipate where the train is routed - the train may today be routed against expectations due to a blockage, signaller error or driver habituation. If this is the case, the driver will choose exactly the right permitted speed, for exactly the wrong route. Hence junction signalling informs the driver of the route, and the driver then uses route knowledge to interpret the right speed.

So, (diverging) junctions do usually involve permanent speed restrictions, but they are often not referred to as such as it's assumed that the junction signalling provides adequate mitigation of the PSR.

At a converging junction, there is no choice of route, and hence the signal aspects are not used to remind the driver - total reliance upon route knowledge.