PSR

[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......