How are speed limits set?

[Irascible]

Speed limits around bends are set for passenger comfort. Hence when BR resarch showed that it was possible for trains to go 40% faster round the bends without risk of derailment, they proposed the Advanced Passenger Train with tilt so that it could go 40% faster round the bends.

Technically that's cant deficiency ( the APT showed you don't want 0 cant deficiency because it made people a bit uncomfortable ). There's RSSB documents defining limits & discussing the matter, as usual.

 

[ptreanor]

How long the blocks are , are based on the speed/stopping distance are they not?

That's one of factors.

What comes first? Do we say "we want a 100mph railway, design the signals to cope with it" Or do we say "This is where the signals are, how fast can we go?"

Both of these factors are taken into account. The distance between the first caution (yellow) aspect and the associated stop signal should not be less than service (normal) braking distance.

 

[deltic08]

Almost certainly not. If such a thing did exist, how would it account for cant?

Not impossible to calculate. Just have two scales, one without cant and one with various degrees of cant to 4%.

 

[Railsigns]

Not impossible to calculate. Just have two scales, one without cant and one with various degrees of cant to 4%.

That's great. Now all we need is an Ordnance Survey map that shows the cant of every curve.

 

[deltic08]

That's great. Now all we need is an Ordnance Survey map that shows the cant of every curve.

Being purile is unnecessary on this site. The cant levels would be on a gauge not the map.

 

[Railsigns]

Being purile [sic] is unnecessary on this site. The cant levels would be on a gauge not the map.

I'm sorry, but your original question was about the viability of determining the maximum speed over a curve using only a gauge laid on an OS map. You can't determine the maximum speed of a curve without knowing both the radius and the cant.

 

[Bald Rick]

Not impossible to calculate. Just have two scales, one without cant and one with various degrees of cant to 4%.

… and various degrees of cant deficiency.

(Pedant alert - cant and cant deficiency on the NR network are measured in mm, or degrees, but not %)

 

[snowball]

(Pedant alert - cant and cant deficiency on the NR network are measured in mm, or degrees, but not %)

However, for standard gauge track, degrees of cant (or of CD) is almost the same thing as inches of cant (or of CD), because there are 57.3 degrees in a radian, which is almost the same as the gauge in inches.

 

[CEN60]

I’ll save you all the OS Pain - it is not accurate enough to calculate an accurate speed (or even indicative speed) in fact the OS grid system is not accurate enough for long track alignments, hence the boffins at UCL created a thing called Snakegrid. Also for the cant pedants above 25mm of cant is approx 1 degree of rotation of the rails across the track gauge! NR do not have an OS Map with the cant of every curve. However NR in Scotland have programme of running trains with vehicle mounted laser scanners, from which the rails are digitised and strung into strings of Digital CAD points - these all can have radii and existing cant extracted for use in speed calculations.

 

[mcmad]

I'm sorry, but your original question was about the viability of determining the maximum speed over a curve using only a gauge laid on an OS map. You can't determine the maximum speed of a curve without knowing both the radius and the cant.

You can determine the maximum theoretical speed of any curve by using maximum cant and deficiency values.

 

[snowball]

You can determine the maximum theoretical speed of any curve by using maximum cant and deficiency values.

Which is part of what I said in #26.

 

[CEN60]

You can determine the maximum theoretical speed of any curve by using maximum cant and deficiency values.

The max theoretical speed also depends on material type also (rail type / sleepers / ballast depth)

 

[deltic08]

… and various degrees of cant deficiency.

(Pedant alert - cant and cant deficiency on the NR network are measured in mm, or degrees, but not %)

Yes I knew that. Why I said percentage I don't know.
Is It possible to determine a speed of curve from a map. I asked the original question because the overall linespeed from Leeds to Harrogate has been 60mph for ever even though heritage DMUs would exit out of the bottom of Bramhope tunnel doing 70mph and hold this speed before braking at Pannal for Crimple Jnc.
In the 1970s I rode in the cab of a late running Deltic+8 empty stock move from Neville Hill for a morning Harrogate-London train. We travelled up to Horsforth at over 70(then 45mph psr at Horsforth) and exited Bramhope at over 80mph. The ride was smooth and with no banging flanges
The Leeds-Harrogate line was resignalled in 2013 with more sections and better signal sighting for shorter headways and increased capacity. It was also claimed in the local media that it was signalled for a linespeed of 75mph. Still no hint of an increase in speed.
What I want to know is will the present curvature+cant allow 75mph regardless of past illegal speeds on the route?

 

[CEN60]

Yes I knew that. Why I said percentage I don't know.
Is It possible to determine a speed of curve from a map. I asked the original question because the overall linespeed from Leeds to Harrogate has been 60mph for ever even though heritage DMUs would exit out of the bottom of Bramhope tunnel doing 70mph and hold this speed before braking at Pannal for Crimple Jnc.
In the 1970s I rode in the cab of a late running Deltic+8 empty stock move from Neville Hill for a morning Harrogate-London train. We travelled up to Horsforth at over 70(then 45mph psr at Horsforth) and exited Bramhope at over 80mph. The ride was smooth and with no banging flanges
The Leeds-Harrogate line was resignalled in 2013 with more sections and better signal sighting for shorter headways and increased capacity. It was also claimed in the local media that it was signalled for a linespeed of 75mph. Still no hint of an increase in speed.
What I want to know is will the present curvature+cant allow 75mph regardless of past illegal speeds on the route?

After a recent change to the relevant NR Standard unless you know the type and age of rail, sleepers, sleeper spacing, clips etc - you cannot calculate the theoretical maximum speed a curve with a set amount of cant can achieve.

 

[snowball]

After a recent change to the relevant NR Standard unless you know the type and age of rail, sleepers, sleeper spacing, clips etc - you cannot calculate the theoretical maximum speed a curve with a set amount of cant can achieve.

However, all those are things that are far easier and cheaper to change than the radius of the curve. Changing the radius of the curve is likely to involve land purchase and statutory processes.

 

[CEN60]

However, all those are things that are far easier and cheaper to change than the radius of the curve. Changing the radius of the curve is likely to involve land purchase and statutory processes.

Correct - but the issue was calculating speed from a map - my statement still stands - ie a map does not give you all the info you need.

 

[Lurcheroo]

What comes first? Do we say "we want a 100mph railway, design the signals to cope with it" Or do we say "This is where the signals are, how fast can we go?"

We say the former (and we want this timetable with these trains so make sure it can accommodate that too please).

Out of curiosity, surely if a brand new rail line was being built and it was planned for a certain end to end running time then it would be determined what speeds would need to be achieved and therefore what sort of signal spacing you need to have ‘service breaking distance’, as well as some other factors like wether you have 3 or 4 aspect signalling and signal sighting.

But if you were just looking to improve journey times on an already built line with functional signalling then you use the “this is where the signals are, how fast can we go” method ?

Don’t work in signalling at all, just my electrical engineer brains way of thinking so apologies if it’s totally off!

 

[CarrotPie]

Out of curiosity, surely if a brand new rail line was being built and it was planned for a certain end to end running time then it would be determined what speeds would need to be achieved and therefore what sort of signal spacing you need to have ‘service breaking distance’, as well as some other factors like wether you have 3 or 4 aspect signalling and signal sighting.

But if you were just looking to improve journey times on an already built line with functional signalling then you use the “this is where the signals are, how fast can we go” method ?

Don’t work in signalling at all, just my electrical engineer brains way of thinking so apologies if it’s totally off!

Essentially, yes, unless there's unless money around to change that (or, more likely, remodel junctions and stations to achieve a similar effect).

 

[Lurcheroo]

Essentially, yes, unless there's unless money around to change that (or, more likely, remodel junctions and stations to achieve a similar effect).

Ahh ok that’s cool ! Yeah of course, kind of forgot that it’s possible to re-signal an area if it’s needed as part of the improvements haha !
Thanks for the info

 

[deltic08]

After a recent change to the relevant NR Standard unless you know the type and age of rail, sleepers, sleeper spacing, clips etc - you cannot calculate the theoretical maximum speed a curve with a set amount of cant can achieve.

The majority of the line between Horsforth and Harrogate was relaid with cwr on concrete sleepers between 1970-1976 when I was a daily commuter. As Harrogate-London trains are no longer loco-hauled and other services predominantly DMU operated, I presume stress and wear on the rail is much less on curves than it was so curving speed could be higher on current track. Is that logical?

 

[Annetts key]

Ahh ok that’s cool ! Yeah of course, kind of forgot that it’s possible to re-signal an area if it’s needed as part of the improvements haha !
Thanks for the info

In many cases, the documentation for the signalling system already has enough information for the maximum speed to be calculated.

For lines that existed before colour light signalling was installed, the speed is determined by money. As in the amount of investment that was used to build the line, the cost of on going maintenance and the type of trains using the line (no point designing, building and maintaining a line for 110MPH if the fastest trains normally using it are limited to 75MPH).

Lines with higher speeds cost more in both design of the permanent way and the cost of on going maintenance.

So for existing lines, the signalling is designed for the current requirements within the limits of the permanent way and taking certain other things into account (e.g. junctions, terminal or complex stations). The signal spacing and type (two aspect, three aspect, four aspect) is also determined by the type of train (stopping services, fast/express services, high speed services, freight) and how many trains per hour have been specified for that section of line.

There are solutions for signals that may have insufficient sighting distance (banner signals for example). Although an alternative signal position may also be considered, as there may be some "wiggle room".

For new lines, then yes, the requirements will be determined for what is required to meet the specification. Both the permanent way and the signalling will be designed to meet the specification.

Just as the cost of the permanent way is affected by the specified line speed, the cost of the signalling is also partly determined by the line speed. However, the required line capacity (trains per hour) has a bigger effect on costs. You can have a 125MPH line using two aspect signalling. You can't have an intensive commuter line with two aspect signalling if that line is also a high speed line or it has mixed traffic...

When a line or area is resignalled, again, cost is a major factor. A lower cost scheme will often reuse existing signals/structures/signal positions. A more expensive scheme may completely redesign the signal spacing and signal positions. Especially if work is being done to raise line speeds.

 

[Sun Chariot]

Just interested in how speed limits were set.

I'm sure the Victorian mathematicians and civil engineers did the sums - but a wicked part of me imagines them seeing at what point the loco tipped over on the bend and then setting the limit at 5mph less.