Simple signalling question [Answered]

[najaB]

I was out for a walk yesterday afternoon and saw a train heading north (well, east) out of Dundee heading towards Broughty Ferry. It passed signal D1001R which, I assume, repeats the signal protecting the level crossing at Broughty Ferry. What puzzled me is that it acted like an automatic signal - going from green to yellow as soon as the train passed. I would have thought that, since it's a repeater, it would have stayed green until the main signal (D1001) changed.

Is it because the crossing signal is approach controlled and went to yellow after D1001R was passed?

 

[carriageline]

Most repeaters I’ve seen are replaced once a train passes them. I assume, if another train was to follow (IE a SPAD) it will cause that train to start slowing, and not just accelerate as they have a green!

 

[najaB]

Most repeaters I’ve seen are replaced once a train passes them. I assume, if another train was to follow (IE a SPAD) it will cause that train to start slowing, and not just accelerate as they have a green!

Ah, okay. That explains it. Thanks!

I got a little confused because the repeater going onto the Tay Bridge (D764R I think) doesn't - probably because both it and the main signal are visible at the same time if there's no fog.

Edit: D714

 

[Driver2B]

Most repeaters I’ve seen are replaced once a train passes them. I assume, if another train was to follow (IE a SPAD) it will cause that train to start slowing, and not just accelerate as they have a green!

I was trained all must.
However, some of the training isn't true of some other regions.

 

[Tomnick]

A colour light (or indeed motor worked semaphore in some cases!) distant will invariably replace itself to caution, if there's a separate track circuit between there and the associated stop signal, when that track circuit becomes occupied. There is, or at least wasn't, any requirement to provide a track circuit specially for that purpose though. Often you'll see the distant stay off until the train occupies the relatively short berth track circuit in rear of the stop signal.

"Repeater" is a bit of a misnomer in this case, but the same applies to banner repeaters - they'll often return 'on' after the front of the train's passed them, if there's a suitable track circuit. I understand that the theory, as above, is that a second train entering the occupied section legitimately or otherwise whilst the first is standing between the repeater and the stop signal to which it relates, might otherwise be misled into overlooking the presence of the first train.

 

[mr_moo]

I'm not a signalling engineer but I asked a colleague about this a few years back when I noticed the same. IIRC he stated that it's got two purposes:

1) (Depending on how close signals are together) If the train following behind sees the signal at green and empty track between them and it, the driver may focus on that and SPAD their own signal which, of course, will still be at red because the green is only a distant.

2) If a train breaks down or has to stop for an emergency or something, it will often deliberately stop at a main controlling signal to allow for easier contacting of the signaller and location identification. If the failure or emergency requires another train or loco to come and assist, and this comes from behind the failed train, the assisting driver will receive a green aspect followed by the rear end of a train, and thus the chance of a collision is increased. Whilst, being a distant, it's unable to show a red, a yellow is at least more restrictive, and thus if the assisting driver does loose situational awareness, they will at least be going slower, expecting a red, so when they find a train's rear end they are much less likely to crash into it.

Thus, a separate track circuit is usually provided specifically to trigger this reversion to the most restrictive aspect the signal is able to display. It can be omitted by risk assessment I believe, but it's general best practice to include it.

 

[Annetts key]

As Tomnick says. If there is a suitable track circuit or axle counter section, the colour light ‘distant’ will return to it’s most restrictive aspect (single yellow).

Incidentally, not all ‘stop’ colour light signals return to red immediately after a train passes them. Some controlled signals where the shunting of long trains may be required, or in station areas, have ‘last wheel’ replacement. And most automatic signals don’t have a separate berth and overlap track circuit / axle counter section, so stay off until the train goes onto the overlap track circuit / axle counter section.

Hence if there is a ‘distant’ signal for either of the above with no separate suitable track circuit or axle counter section, the ‘distant’ can stay showing off for longer than people expect.

== Doublepost prevention - post automatically merged: 22 Jun 2021 ==

I'm not a signalling engineer but I asked a colleague about this a few years back when I noticed the same. IIRC he stated that it's got two purposes:

1) (Depending on how close signals are together) If the train following behind sees the signal at green and empty track between them and it, the driver may focus on that and SPAD their own signal which, of course, will still be at red because the green is only a distant.

That’s what is known as read through and can be a problem on straight lines, or where the curves and buildings or vegetation allow a signal further ahead to be seen misleading a driver, even if the view is intermittent. This may affect any colour light signal, not just ‘distant’ signals.

2) If a train breaks down or has to stop for an emergency or something, it will often deliberately stop at a main controlling signal to allow for easier contacting of the signaller and location identification. If the failure or emergency requires another train or loco to come and assist, and this comes from behind the failed train, the assisting driver will receive a green aspect followed by the rear end of a train, and thus the chance of a collision is increased. Whilst, being a distant, it's unable to show a red, a yellow is at least more restrictive, and thus if the assisting driver does loose situational awareness, they will at least be going slower, expecting a red, so when they find a train's rear end they are much less likely to crash into it.

Thus, a separate track circuit is usually provided specifically to trigger this reversion to the most restrictive aspect the signal is able to display. It can be omitted by risk assessment I believe, but it's general best practice to include it.

Yes, it’s best practice and modern practice for there to be a controlling track circuit/axle counter for this purpose. But not all systems were implemented this way in the past, and, unless there have been problems, generally, retrospective action is not usually required.

 

[Dougman]

It used to have a red aspect too, which is why it has a TC.

The signal in question, and the signal opposite, are three aspect heads that only show green or yellow, but both used to also show red.

 

[najaB]

It used to have a red aspect too, which is why it has a TC.

The signal in question, and the signal opposite, are three aspect heads that only show green or yellow, but both used to also show red.

Thanks. I had noticed that it was a three aspect head but didn't know it had been changed. Do you have any idea why the change?

 

[Dougman]

Thanks. I had noticed that it was a three aspect head but didn't know it had been changed. Do you have any idea why the change?

No idea I'm afraid. I just noticed that it had changed a long time ago.