31-05-2011, 03:26 PM
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.
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.