Signal Query

[fairysdad]

Earlier on, I was sat in my car in the station car park at Hamworthy station. A train came into the down platform (towards Weymouth), and departed. I noticed that the signal at the end of the platform (HW50) was on green when the train was in the station, but more interestingly, it remained green after the train had passed it.

This is the signal involved - I assume that the semaphore signal behind is for the Hamworthy goods line down to Poole docks (if I didn't see the semaphore, I would've assumed that the green would've been for that line instead).

I'm just curious to know really! I've seen that usually signals turn red as soon as the train passes it (a while ago I was on a train about mid-way along, and somebody was panicking because to them, the train was passing all these red signals...!) so was wondering why this one didn't...

TIA

 

[Railsigns]

It sounds like the signal box was switched out at the time.

 

[455driver]

Did the signal go back to red eventually?
If it did then it all it means is that the block joint is some distance from the actual signal, they can be up to 200 yards (or more) from the signal and until the train goes over the joint the signal will stay green.

The signal in rear will still be red so its all safe (and normal).

A block joint is the thing that separates adjoining track circuits, when the train goes over the block joint onto the next track circuit the relay will drop and the track circuit will show occupied (red), the train will now be showing as being in 2 track circuits, when the last wheel-set leaves the previous track circuit, that one will show white and the previous signal can be cleared.

When the block joint is positioned at the signal there will be another block joint at the end of the overlap so as the train goes over the first joint the signal will go to red but the previous signal will also be held at red until the last axle has cleared the joint at the end of the overlap.

What you have seen is what happens when there is only 1 block joint positioned at the end of the overlap for that signal, it is a simpler/ cheaper way of controlling the signals but is still perfectly safe.

 

[pendolino]

Is it a distant signal? (signal number would be something like AB12R, or there would be a white triangle on the signal plate).

If so, it can only show a green or yellow aspect. It can't show red when the train passes (there's no red aspect to show), but nor can it go to single yellow as that would mean the stop signal ahead must be showing a red aspect (which it can't be if the train passed the distant when it was showing green). So it stays green until the train passes the stop signal ahead which then reverts to red and the distant changes to yellow.

 

[matchmaker]

Is it a distant signal? (signal number would be something like AB12R, or there would be a white triangle on the signal plate).

If so, it can only show a green or yellow aspect. It can't show red when the train passes (there's no red aspect to show), but nor can it go to single yellow as that would mean the stop signal ahead must be showing a red aspect (which it can't be if the train passed the distant when it was showing green). So it stays green until the train passes the stop signal ahead which then reverts to red and the distant changes to yellow.

No. It might be a distant signal at that location, but in view of the semaphore stop signal on the line behind, I'd say it is a stop signal.

The fact that it didn't change after the train passed probably means that the controlling signal box is switched out.

 

[SIGGY56]

No. It might be a distant signal at that location, but in view of the semaphore stop signal on the line behind, I'd say it is a stop signal.

The fact that it didn't change after the train passed probably means that the controlling signal box is switched out.

Some colour light signals have what they call "first wheel replacement" i.e. the first set of wheels on a train replaces the signal to danger when occupying the next track circuit at the signal so the rest of the train passes the signal at red. The other method is "last wheel replacement" the whole train has to pass the signal before the signal goes back. Both methods are standard and quite safe provided the driver doesn't get a change of aspect in front of him - otherwise that could well end being a SPAD then!!.

In automatic signal sections I know few signals that don't go back after the train immediately , that depends on how long the track circuit overlap for the signal is - could be 200, 300 or even 400yds in some cases.

 

[MrC]

Railsigns has the correct answer. Hamworthy box is part time so when it's locked out Poole - Wareham is operated as one block section and Hamworthy's signals are set to greens. This can cause delays as Poole can't let a train enter the section at the Holes Bay relief road flyover until a previous one has got to Wareham.

This will all change with the re-signalling proposed for 2013......

 

[455driver]

Is it a distant signal? (signal number would be something like AB12R, or there would be a white triangle on the signal plate).

If so, it can only show a green or yellow aspect. It can't show red when the train passes (there's no red aspect to show), but nor can it go to single yellow as that would mean the stop signal ahead must be showing a red aspect (which it can't be if the train passed the distant when it was showing green). So it stays green until the train passes the stop signal ahead which then reverts to red and the distant changes to yellow.

No it doesnt, where did you get this information from!

 

[BravoGolfMike]

Is it a distant signal? (signal number would be something like AB12R, or there would be a white triangle on the signal plate).

If so, it can only show a green or yellow aspect. It can't show red when the train passes (there's no red aspect to show), but nor can it go to single yellow as that would mean the stop signal ahead must be showing a red aspect (which it can't be if the train passed the distant when it was showing green). So it stays green until the train passes the stop signal ahead which then reverts to red and the distant changes to yellow.

Sorry but that is simply not true, distance signals revert to their most restrictive aspect after a train has passed, not once it has reached the stop signal... Even modern banner repeaters go back before the associated signal as the train passes.

 

[Railsigns]

There are some sweeping generalisations being made here. Some distant signals are replaced to 'caution' as soon as a train passes them. However, some continue to show green until the stop signal ahead returns to 'danger'. All will be replaced by the first available train detection section (track circuit or axle counter) ahead, which may start immediately beyond the distant signal, beyond the stop signal, or somewhere between the two. Sometimes a treadle may be installed just past the distant signal to replace its aspect to 'caution' in the absence of a track circuit or axle counter boundary at that position.

 

[MarkyT]

Sorry but that is simply not true, distance signals revert to their most restrictive aspect after a train has passed, not once it has reached the stop signal... Even modern banner repeaters go back before the associated signal as the train passes.

Referring to Railway Group standards . . .

Railway Group Standard
GK/RT0060
Issue Four
Date June 2003

Interlocking Principles

http://www.rgsonline.co.uk/Railway_...ng/Railway Group Standards/GKRT0060 Iss 4.pdf

Extracts -

C7.4 Control of distant signals
A distant signal shall only clear from its most restrictive aspect when all the
associated stop signals ahead have been cleared. Where provided, track
sections between the distant signal and the associated stop signal(s) shall be
proved clear. Where this control is not possible, the signaller shall be provided with a means of placing or maintaining the distant signal to its most restrictive aspect (for example, a separate lever or a replacement switch).
A replacement switch shall also be provided for distant signals reading from
absolute block sections or where reading up to stop signals protecting controlled level crossings.
A distant signal shall require the stop signal ahead to be alight. An outer distant (where provided) shall require the inner distant off and alight.

. . .

C8.2.6 Replacement of distant signals
On lines where permissive block working applies, a distant signal shall be
replaced to its most restrictive aspect immediately after the passage of each train.
On other lines, it shall be replaced by occupation of the first available track
section beyond the signal.

- End extract

So except on permissive block it is not neccessary to provide a block joint at the distant specifically to replace it, but the next block joint the train encounters after the distant should replace it. With traditional track circuits, their length restrictions (especially on electrified lines) usually mean there are several sections between signals, so in most cases a distant signal auto-replaces before the train reaches the first stop signal it applies to.

 

[Joseph_Locke]

... and deliberately holding the Distant at green until the track ahead of the Stop occupies isn't ALARP.