How are speed limits calculated?

[Clansman]

As the title says. I have often wondered how speed limits are calculated on lines. Particularly in Scotland where I live, I tend to wonder why certain sections of railway aren't a faster limit. Is there any reason why 125mph working is only ever achieved on Intercity routes (daft it may sound) rather than on commuter routes. I'm mostly on about the routes which serve our 7 cities up here. Sections from Perth to Dunblane for example make me wonder why there isn't a higher speed limited from 100mph when the line looks safe enough to do so (in terms of sharp curves and distance between 2 major stations). Does it come down to the track condition (because looking at the Perth to Gleneagles section on paper looks like it could constitute a higher limit, although the track condition makes for a bumpy ride) if anything or the route purpose itself? For example there are barely any 125mph sections across the national rail network on commuter routes or even those which aren't under the wires (apart from the MML and sections out of Paddington). Surely more could be done to allow speed increases throughout the network?

Just curious, so I request my head may not be bitten off if I sound daft

 

[fowler9]

I wouldn't worry about getting your head bitten off, no such thing as a stupid question. I don't know the exact answer but would like to have a guess. The track alignment where I live in South Liverpool would probably allow faster running on Liverpools WCML spur, however, given the number of services on the route that would prevent that happening anyway it makes more sense financially not to maintain the permanent way to the level required to allow 125 running. I am guessing this holds true for many places. I am guessing with enough investment the Cheshire Lines Route between South Parkway and Manchester could cope with 125 mph running but the figures don't add up for when it would be needed. Just my thoughts.

 

[Domh245]

My understanding is that it is often the signals that are one of the biggest factors when it comes to line speed. The track alignment may be good for higher speeds, but those higher speeds will need longer signal blocks and overlaps, resulting in fewer signal blocks and thus lower capacity. Those signalling considerations are then based on the typical stock and service that is expected to operate. A bit Chicken and Egg really.

 

[Joseph_Locke]

Track curvature, signal spacing, level crossings, train stopping patterns, track construction, train capabilities, presence or absence of TASS overlays, structure clearances, formation stiffness, earthworks condition, aerodynamic effects in tunnels, transition length, vertical curves, cant excess issues, negative cant, OLE wire gradient, presence of DC conductor rail, presence of complex S&C, presence of jointed track, presence of longitudinal timbers, limited signal sighting, speed signage design constraints, driveability considerations, limited signal overlap lengths and junction signalling controls are a few I can think of.

And by the way, it is permissible speed, not linespeed. NR no longer publishes speeds by exception and all changes in speed are (or should be) indicated on by signs.

 

[edwin_m]

On the specific example of Perth to Dunblane there is only one train each way per day that is capable of exceeding 100mph. Increasing speed involves extra cost to upgrade various things, including greater clearance for staff refuges above 100mph, and also extra maintenance costs as it may put the line into a higher category for inspection frequency. This is only worthwhile if there are enough trains able to take advantage of it, and presumably Network Rail has to recover this cost by a higher access charge.

 

[Tio Terry]

As the title says. I have often wondered how speed limits are calculated on lines. Particularly in Scotland where I live, I tend to wonder why certain sections of railway aren't a faster limit. Is there any reason why 125mph working is only ever achieved on Intercity routes (daft it may sound) rather than on commuter routes. I'm mostly on about the routes which serve our 7 cities up here. Sections from Perth to Dunblane for example make me wonder why there isn't a higher speed limited from 100mph when the line looks safe enough to do so (in terms of sharp curves and distance between 2 major stations). Does it come down to the track condition (because looking at the Perth to Gleneagles section on paper looks like it could constitute a higher limit, although the track condition makes for a bumpy ride) if anything or the route purpose itself? For example there are barely any 125mph sections across the national rail network on commuter routes or even those which aren't under the wires (apart from the MML and sections out of Paddington). Surely more could be done to allow speed increases throughout the network?

Just curious, so I request my head may not be bitten off if I sound daft

As well as signalling issues, if you raise linespeeds above 100mph there is also the need to improve lineside clearances for staff as for linespeeds above 100mph you have to be able to stand 2M clear of the closest running line. There were issues with this for linespeed increases between St Pancras and Sheffield.

I'm not certain - somebody on here will know - but I believe there is also a change in Maintenance requirements when speeds exceed 100mph.

 

[QueensCurve]

Track curvature, signal spacing, level crossings, train stopping patterns, track construction, train capabilities, presence or absence of TASS overlays, structure clearances, formation stiffness, earthworks condition, aerodynamic effects in tunnels, transition length, vertical curves, cant excess issues, negative cant, OLE wire gradient, presence of DC conductor rail, presence of complex S&C, presence of jointed track, presence of longitudinal timbers, limited signal sighting, speed signage design constraints, driveability considerations, limited signal overlap lengths and junction signalling controls are a few I can think of.

And by the way, it is permissible speed, not linespeed. NR no longer publishes speeds by exception and all changes in speed are (or should be) indicated on by signs.

Thank you for a comprehensive list.

For clarity, what are "speeds by exception"?

 

[Joseph_Locke]

Thank you for a comprehensive list.

For clarity, what are "speeds by exception"?

How it used to be:

"All Fast lines 110mph unless shown otherwise.
All Slow lines 90moh unless shown otherwise.

All lines 0m 0c to 1m 20c 15mph.
Fast lines from 18m 60c to 19m 20c 80mph both directions, etc."

Now it would be 9for the Down Fast, e.g.:

"0m 0c to 1m 20c 15mph
1m 20c to 18m 60c 110mph
18m 60c to 19m 20c 80mph
19m 20c to .... 110mph"

I'm not certain - somebody on here will know - but I believe there is also a change in Maintenance requirements when speeds exceed 100mph.

It varies with track category (6 to 1A) which is based on annual tonnage and maximum permissible speed: see page 105 of NR/L2/TRK/2012.

 

[Clansman]

On the specific example of Perth to Dunblane there is only one train each way per day that is capable of exceeding 100mph. Increasing speed involves extra cost to upgrade various things, including greater clearance for staff refuges above 100mph, and also extra maintenance costs as it may put the line into a higher category for inspection frequency. This is only worthwhile if there are enough trains able to take advantage of it, and presumably Network Rail has to recover this cost by a higher access charge.

Projecting forward to 2019 that number will be over 20x more for 125mph capable stock. When the HSTs come up here there will be around 2 trains per hour in each direction diagrammed for them. This outweighs the services undertaken by 100mph stock by 2:1 (in each direction) when new Dundee to Glasgow services take effect by 2019 also, run by Turbostars. Even excluding the IEPs you could say that there is a case for upgrading the line for the HSTs. Unless the HSTs will be re-geared to run at 100mph, in which case the idea of upgrading the line wouldn't work?

The same could be said for the St. Madoes to Invergowrie stretch of the Perth to Dundee line, where the line undertakes no severe curves and is basically straight over the 10 mile stretch.

 

[W230]

...you could say that there is a case for upgrading the line for the HSTs.

There just isn't the drive/need though. As mentioned a lot of cost involved and for what benefit? A couplf of minutes shaved off a journey here and there?

While I don't know the areas you mention, it's normally better to improve areas where you have the minimum permissible speeds (say a 15 mph section for maybe 40 mph during a replacing/reworking of points etc or maybe 'flashing yellow' junctions in place of approach control from red).

 

[Clansman]

There just isn't the drive/need though. As mentioned a lot of cost involved and for what benefit? A couplf of minutes shaved off a journey here and there?

While I don't know the areas you mention, it's normally better to improve areas where you have the minimum permissible speeds (say a 15 mph section for maybe 40 mph during a replacing/reworking of points etc or maybe 'flashing yellow' junctions in place of approach control from red).

I see your point there. Areas including the approach to Perth, Dundee ,Aberdeen and Inverness could be candidates for the kind of speed increase you highlighted.

A bit off topic, but foes the Scottish Route Study say anything about line upgrades for electrification works in the near future? I just think that if you are electrifying the line you'd be as well make track infrastructure improvements on it also.

My understanding is that it is often the signals that are one of the biggest factors when it comes to line speed. The track alignment may be good for higher speeds, but those higher speeds will need longer signal blocks and overlaps, resulting in fewer signal blocks and thus lower capacity. Those signalling considerations are then based on the typical stock and service that is expected to operate. A bit Chicken and Egg really.

I understand what you are saying but I can't help wonder how this explains non-capable 125mph stock running on fast lines at regular intervals. For example, lines such as Chathill to Newcastle, Edinburgh to Drem and London to Peterborough come to mind - where non 125mph stock run on the same line as those which regularly maximise line speed.

 

[Railsigns]

Areas including the approach to Perth, Dundee Aberdeen and Inverness could be candidates for the kind of speed increase you highlighted.

The present track layouts at Dundee and Inverness both date from the second half of the 1980s and already contain a good number of crossovers and turnouts fit for 40 mph. Those with lower speeds are likely to be constrained by the physical characteristics of the sites.

Aberdeen's layout dates from the early 1980s and looks as though it might benefit from some higher speed turnouts.

Perth's steam age layout needs ripping up and starting again.

 

[richieb1971]

Wouldn't you need more freight loops for 125mph on dual tracks?

North of Bedford the fasts have all the money spent on them. The slows which carry freight only are much more rickety. I'd like to know what the slows speed limit will be post electrification. The up slow at Oakley has a 20mph speed limit due to landslip potential and I Can't see this sticking if passenger services use it after electrification. Will be interesting.

 

[Domh245]

I understand what you are saying but I can't help wonder how this explains non-capable 125mph stock running on fast lines at regular intervals. For example, lines such as Chathill to Newcastle, Edinburgh to Drem and London to Peterborough come to mind - where non 125mph stock run on the same line as those which regularly maximise line speed.

That is something that tends to be covered by paths and timetabling. You would send a 125mph train through, and then send a slower train immediately (as reasonably practical) behind it, which then hopefully clears the section before it is caught by the next fast train. And don't forget that if you have a Turbostar being chased by an HST, both at top speed, there is only a 25mph speed difference between them, which isn't that much.

 

[70014IronDuke]

That is something that tends to be covered by paths and timetabling. You would send a 125mph train through, and then send a slower train immediately (as reasonably practical) behind it, which then hopefully clears the section before it is caught by the next fast train. And don't forget that if you have a Turbostar being chased by an HST, both at top speed, there is only a 25mph speed difference between them, which isn't that much.

25 mph speed differential "isn't that much" ?

I ain't no timings expert, but I'd say it's a massive difference if you've got that on one of today's intensively worked railways over anything but minimal distances. :cry:
I await the response of a specialist.

 

[Ianno87]

25 mph speed differential "isn't that much" ?

I ain't no timings expert, but I'd say it's a massive difference if you've got that on one of today's intensively worked railways over anything but minimal distances. :cry:
I await the response of a specialist.

Depends. All other things being equal, and over a sufficiently long and intensively utilised section, then it will.

Shap is a classic example. On the face of it, a 100mph Desiro should get run down by a 125mph Pendolino, but what the Desiro lacks in top speed it largely compensates for in superior acceleration, and speedier dwell times. The flighting of freight services over Shap which are *much* slower than passenger services is the overall driver of capacity.

Flip side is something like the WCML Fast Lines out of Euston, where departures have to be sequenced in a very particular order to get the right service intervals and stoppibg patterns whilst accounting for the mix of 100/110/125mph. But making *everything* 125mph wouldn't necessarily buy all that much more capacity - stops at Watford or crossing moves at Ledburn would start to become the limiting factors, or once you want to start mixing stopping patterns further north. But you can do this because Euston has oodles of platform capacity to play around with to sequence trains into the optimal departures order.

It is only when you have a pretty much operational constraint-free railway (as HS2 is likely to be, at least off the NR network, such as grade separation of all junctions) that you can "max out" with homogenised train speeds. But even then (at least until Phase 2 is finished) there are only *so* many trains that can sensibly be accommodated at Liverpool Lime St, Manchester Piccadilly, etc without majorly impacting local/regional services.
--- old post above --- --- new post below ---

There just isn't the drive/need though. As mentioned a lot of cost involved and for what benefit? A couplf of minutes shaved off a journey here and there?

While I don't know the areas you mention, it's normally better to improve areas where you have the minimum permissible speeds (say a 15 mph section for maybe 40 mph during a replacing/reworking of points etc or maybe 'flashing yellow' junctions in place of approach control from red).

Low speed junctions/speed profile dips are the low-hanging fruit, wheeras upgrading already higher speed lines has diminishing returns. For example, running at 110mph vs 100mph saves about 3 seconds per mile. So you need to upgrade 20 miles continuously to save a whole 1 minute of journey time. And any train stopping at a station in this 20 mile section will only receive about half the benefit or so.

 

[najaB]

Low speed junctions/speed profile dips are the low-hanging fruit, wheeras upgrading already higher speed lines has diminishing returns. For example, running at 110mph vs 100mph saves about 3 seconds per mile. So you need to upgrade 20 miles continuously to save a whole 1 minute of journey time. And any train stopping at a station in this 20 mile section will only receive about half the benefit or so.

Not to mention that upgrading that 20 miles only benefits stock that's capable of 110mph, whereas removing a 20mph PSR will benefit every service.

 

[Bald Rick]

To add to Mr Locke's excellent responses, and to replay something I write on this forum every few months (it seems), there are well over 80 (and rising) separate infrastructure factors that can affect line speeds. Only one of them is the designed track geometry. All have to be assessed before a linespeed can be raised.

As it happens I was driving on the old A9 near Kingussie a few days ago, and saw a 100mph sign on the adjacent Highland line. I didn't realise the linespeed was that high.

 

[notadriver]

Of course Desiros aren't allowed to do the EPS speeds and whilst they have quicker dwell times the Pendolinos have less stops.

As far as Shap is concerned a Pendo is still faster than a Desiro on the climb and I dare say a pendolino accelerates better than a Desiro above 60 mph.

 

[Ianno87]

Of course Desiros aren't allowed to do the EPS speeds and whilst they have quicker dwell times the Pendolinos have less stops.

As far as Shap is concerned a Pendo is still faster than a Desiro on the climb and I dare say a pendolino accelerates better than a Desiro above 60 mph.

From a standing start at Oxenholme, 350s are timed for 10 minutes to Tebay, 390s are timed for 11.

 

[notadriver]

From a standing start at Oxenholme, 350s are timed for 10 minutes to Tebay, 390s are timed for 11.

And your point is ????


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[Ships]

And your point is ????


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That a Pendolino isn't faster over sharp.....

 

[notadriver]

That a Pendolino isn't faster over sharp.....

I wasn't talking about from a standing start. It can use EPS speeds and performance logs show it to be faster on the climb of *Shap* itself.



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[D1009]

Actually the 125 thing isn't the issue in my book until other than Intercity TOCs get stock capable of that speed. I think the upgrading of the Derby Birmingham route to 125 was largely a waste of money as northbound voyagers normally catch up the 170s around Burton, and southbound ones follow all manner of stuff from Water Orton onwards, so the theoretical 30 minute journey time between the two cities is rarely if ever achievable.

In the 1960s the maximum permissible speed between Wolverhampton and Shrewsbury was 90. This was reduced to 70 when most of the trains became DMU as this was their maximum, BR claiming reductions on maintenance costs. AFAIK this remains to this day even though all passenger trains passing that way are capable of 90 or 100, and even the Cambrian has a higher permissible speed.

 

[The Planner]

And they were right, it did save on maintenance. The reason it isn't back up to 90 (we have tried lots of times...) is that the work needed to be done to do that is now hideously expensive compared to what you get back.

 

[QueensCurve]

The present track layouts at Dundee and Inverness both date from the second half of the 1980s and already contain a good number of crossovers and turnouts fit for 40 mph. Those with lower speeds are likely to be constrained by the physical characteristics of the sites.

Aberdeen's layout dates from the early 1980s and looks as though it might benefit from some higher speed turnouts.

Perth's steam age layout needs ripping up and starting again.

Dundee was comprehensively remodelled and resignalled in 1985. The gradients were eased and the main routes eased to 40mph. Dock St tunnel retains a 15mph limit an the junction for the Tay Bridge 25mph reflecting the speed limit on the North end of the bridge itself.

Perth does indeed need ripping up and starting again.

 

[Joseph_Locke]

And they were right, it did save on maintenance. The reason it isn't back up to 90 (we have tried lots of times...) is that the work needed to be done to do that is now hideously expensive compared to what you get back.

Quite. A typical Journey Time Improvement scheme now ends up picking up bridge strengthening works, remedial formation treatment, track componentry upgrades, track quality tamping and, on occasion, signal sighting and structure gauging. At between £500k and £25m per minute saved it is surprising what little can be justified with a business case.

As a previous poster pointed out, addressing short, low restrictions is always first choice.

 

[QueensCurve]

That a Pendolino isn't faster over sharp.....

Oxenholme to Tebay does not include Shap bank.

I seem to recall someone saying that a 350 out-accelerates a Pendolino up to 90mpg but then the 390 gets ahead.

 

[louis97]

From a standing start at Oxenholme, 350s are timed for 10 minutes to Tebay, 390s are timed for 11.

The extra minute for a 390 is an allowance, the sectional running time for a 350 and a 390 is 10 minutes. However 390s have an agreed allowance that has to be applied as per the train planning rules.

 

[The Planner]

All trains should have 1 minute engineering allowance approaching Tebay, not just 390s.