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Reg - Aspect Step up & Step down
#1
Dear Members,

The below statement I read in one training material.

It is an unwritten assumption that an aspect may at any time "step up" to a less restrictive aspect due to the clearing of signals beyond, but in normal circumstances will not step down to a less restrictive aspect.

I am not understand the text which is available in bold.

Please clarify.
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#2
Hi

I wrote those words, so I had better explain them!

On the roads, traffic lights are used as "flow control valves", which routinely revert, via amber, to red, to stop a continuous flow of traffic, to allow the other road its turn. This is quite normal.

However, railway signals are expected to default to red, and only to show a proceed aspect when specifically requested, by the signaller setting a route, to take that train to its desired destination. Signals automatically revert to red immediately behind each train. This means that a train driver approaching a signal will either:

1 Find the signal already at red, because no route has not yet been set, or
2 Find the signal at proceed, because the route has been set for that train's required destination

Threfore, the only circumstances when a driver will encounter a signal becoming more restrictive on the approach are exceptional:

a an equipment fault (track failure) causing aspect reversion
b the signaller made a mistake and wishes to change the set route
c an emergency has been reported and the signaller wishes to attempt to stop the train, without setting another route

You might ask "what about automatic signals, surely they're normally at proceed?"

Yes, they often show proceed, but their default state is red, and they are only allowed to show proceed when the trackside conditions are positively proved safe. Because there is only one route from an automatic signal, there is no doubt about the intended destination of any train approaching the signal, so its OK for the signal to automatically show proceed without signaller action. Therefore the same statement also applies to automatic signals.

Going back to where we started, the whole sentence came from an explanation of aspect sequence charts. The solid lines on an aspect sequence chart describe the static conditions, at one moment in time only. The only branching of the solid lines is usually after a green aspect: where the driver can then discover whether the next signal is either another green, or a first caution.

If we were to populate the chart with arrows showing how aspects might "step up" (routinely) or "step down" (exceptionally), the chart would become confusing. Therefore, the chart only shows the static conditions, and the reader is expected to infer the effects of step up/ step down events, by jumping to the applicable new static line.

Reuben




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#3
From the operational point of view the above is correct. From a technical point of view, signals are designed to "step-down" usually upon a train passing through a section of railway. A driver should not see aspects step nor climb down. In the UK (mainline) that would be classed as an abnormal aspect sequence and requires a driver to stop at the next available safe place to report it.
Le coureur
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#4
I know what you mean, but we would not use that term; we'd say they were replaced or revert to danger. It is not just a step, more of a complete fall!

Interestingly, as we discussed on Saturday, I'd refer to the yellow, double yellow, flashing yellow, green etc as "better aspects" but never call the red the "worst aspect"!

(05-04-2012, 09:52 AM)Jerry1237 Wrote: From the operational point of view the above is correct. From a technical point of view, signals are designed to "step-down" usually upon a train passing through a section of railway. A driver should not see aspects step nor climb down. In the UK (mainline) that would be classed as an abnormal aspect sequence and requires a driver to stop at the next available safe place to report it.

PJW
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#5
Arguably it isn't the worst aspect (black or unexpected more restrictive aspect springs to mind). Is the most restrictive aspect appropriate to red?
Le coureur
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#6
(05-04-2012, 06:51 PM)Jerry1237 Wrote: Arguably it isn't the worst aspect (black or unexpected more restrictive aspect springs to mind). Is the most restrictive aspect appropriate to red?

I'd say so. However, the term is relative to the signal in question. I don't know of a written definition, but the general use that I know of the term is for the aspect that you would like that signal to show if something is wrong. If you have a two aspect Y/G, the most restrictive aspect would be Y - the one that would be fed by the back contacts in RRI, or connected to Red Retaining output on SSI.
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#7
A driver should not see aspects step nor climb down. In the UK (mainline) that would be classed as an abnormal aspect sequence and requires a driver to stop at the next available safe place to report it.



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#8
(04-11-2013, 07:53 AM)zohanx747 Wrote: A driver should not see aspects step nor climb down. In the UK (mainline) that would be classed as an abnormal aspect sequence and requires a driver to stop at the next available safe place to report it.

A driver should not see aspects step down in nomal circumstances; it does happen in failure conditions (things need to fail safe) and in emergency. Occasionally a signaller will inadvertantly replace a signal just prior to a train passing it, or when they think the train is far enough away not to see an aspect reversion but misjudges- therefore it occasionally does happen.

Of course drivers should see aspects step up as conditions improve ahead. Otherwise haveing once shown a driver a Red then you are saying we should never show a Yellow or a Green for example!
PJW
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