Why would a signal fail to clear?

[Rana12345]

Why would a signal fail to clear (remain at danger) when setting a route and the route lights show up and the track circuits are clear?

 

[DarloRich]

Is this an exam question?

 

[Bertone]

A route point detection failed or signal ahead lamp failed, to name just two possibles.

 

[Annetts key]

Why would a signal fail to clear (remain at danger) when setting a route and the route lights show up and the track circuits are clear?

How long have you got? Depending on where it is, there could be dozens or more reasons. The route lights only tell you that the route locking should have been activated. There are far more controls in the signal control circuits.

 

[LCC106]

Welcome to the forum. Can I ask, is this a question you have as part of your training? If so, why are your thoughts on the subject? If it’s general interest then I’m sure some members will be able to help. If it’s the former and you simply don’t know, it may be worth talking it through with your trainer. That’s what they’re there for.

 

[Islineclear3_1]

How is the area signalled? AB, TCB, axle counter etc...?

 

[ComUtoR]

Is the track circuit "occupied when clear" or "clear when occupied" ?

I've seen reports of both circumstances. Could there be a track circuit failure of some sort ?

( I'm not a signaller or S&T)

 

[cin88]

Is the track circuit "occupied when clear" or "clear when occupied" ?

I've seen reports of both circumstances. Could there be a track circuit failure of some sort ?

( I'm not a signaller or S&T)

If a track circuit is showing clear when occupied, the signal protecting it would be able to be cleared.

 

[king_walnut]

Welcome to the forum. Can I ask, is this a question you have as part of your training? If so, why are your thoughts on the subject? If it’s general interest then I’m sure some members will be able to help. If it’s the former and you simply don’t know, it may be worth talking it through with your trainer. That’s what they’re there for.

I've never understood this attitude towards trainees asking questions on this forum. There is literally nothing wrong with helping a person learn.

 

[High Dyke]

Approach controlled signal?

 

[zwk500]

I've never understood this attitude towards trainees asking questions on this forum. There is literally nothing wrong with helping a person learn.

The issue is that the forum may give an incorrect answer in good faith, an answer that has since been superseded, or an answer that is correct for their company/work/area but not for the trainees.

 

[Annetts key]

Is the track circuit "occupied when clear" or "clear when occupied" ?

I've seen reports of both circumstances. Could there be a track circuit failure of some sort ?

( I'm not a signaller or S&T)

Track circuits actually detect the absence of trains (wagons, coaches, cars, locos, whatever). Hence with no train present, they “show clear”.

If a train is present (or indeed anything else that forms a short circuit/low resistance path between the two rails), then the track circuit will be “occupied” and it will show as red lights (or equivalent) in a power signal box/panel/signalling centre.

As they are designed to be as fail save as possible, 99.9999% of failures will cause the track circuit to show occupied.

If a track circuit is occupied, the interlocking will hold one (or more) signals at red to protect the affected section, as the interlocking has to treat it as if it’s a train.

 

[Rana12345]

How is the area signalled? AB, TCB, axle counter etc...?

TCB

 

[Annetts key]

TCB

Can you supply any other information?

Is this a two aspect area, a three aspect area or a four aspect area?
Is this a plain/straight line, or are there points for a junction, loop, or sidings? If yes, what is the layout? What junction or route indicators does the signal have? Which route is being called?
Is there a level crossing involved?
Is there a banner for this signal? Or for the next signal?
Does this signal protect anything unusual? For example, some long tunnels may have an emergency system to put the ‘protecting’ signals to red.
Do you know which type of interlocking system controls the signal?

 

[Islineclear3_1]

Is it an automatic signal or a controlled signal?
Is it on a mainline or loop/siding?
Has the signal bulb/filament "gone/blown"?

If you were the signaller, how would you know if a signal failed to clear?
Do you have a rulebook? Have you read it?

 

[Annetts key]

Is it an automatic signal or a controlled signal?

You normally don’t have route lights for an automatic signal (but I’m sure somewhere there is an exception to that.. ).

 

[rower40]

Displaced Track Circuit Interrupter on a nearby trap point.
Mis-reading control to prevent signal clearing when a different train is approaching an adjacent red signal on the same gantry.
Back-to-Back control to prevent two trains departing the same platform in both directions.
Approaching train has already passed the "flashing aspect inhibit point" but has not yet reached the approach-control (MAR) point
Airport Trip Wire tripped.

Another speciality is this one: the route has failed to release behind the previous train, so it looks as though it has been set for this train, and the signal has stayed at red. Oft-times in ARS areas (when the signaller is not habitually requesting every route) this one occurs, and the signaller assumes that ARS set the route (because the signaller knows he/she did not), but the signal is still red.

I think the forum collective can come up with some even more obscure reasons than these. I look forward to reading them.

 

[edwin_m]

Did anyone mention lamp proving of the next signal?

In SSI-speak at least, these conditions are collectively known as aspect controls. 30 years ago now, I don't recall if approach control was part of aspect controls or had its own category.

 

[carriageline]

Module failure
Junction indicator / theatre box lamp proving failure

A relay fault, if so equipped.

 

[Bertone]

Did anyone mention lamp proving of the next signal?

In SSI-speak at least, these conditions are collectively known as aspect controls. 30 years ago now, I don't recall if approach control was part of aspect controls or had its own category.

Yes me, in the third post in this thread yesterday.

 

[CAF397]

TPWS failed at the next signal which holds the signal in rear at danger.

 

[Gloster]

I don’t think anyone has mentioned a fairly simple reason: the signalman forgot to clear the signal, thought s/he had cleared it but hadn’t or forgot to put it back on automatic after it was in manual, etc.(We’ve all done it, but I did it in the days when all you got was an angry driver saying, “Oi, bobby, when are you going to clear this fiddling signal?”)

 

[Efini92]

TPWS failed at the next signal which holds the signal in rear at danger.

Is that a fact?

 

[liamf656]

Is that a fact?

If I were the trainee I’d take anything said in this forum as a suggestion rather than fact…

The issue is that the forum may give an incorrect answer in good faith, an answer that has since been superseded, or an answer that is correct for their company/work/area but not for the trainees.

…for this reason.