29-03-2012, 12:25 PM
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
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