Distant Signal Locking

andrew jukes
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Distant Signal Locking

Postby andrew jukes » Tue Mar 24, 2009 8:21 pm

I am trying to sort out how distant signals are meant to work so as to be able to simulate the operation of a semaphore signalled block within a computer based interlocking, controlled on an IECC style computer screen - an odd mixture you may think but it happens to be what I want!

Some things are clear enough. A distant must be prevented from clearing if any of the stop signals in the block ahead are at danger, so it must be locked at caution unless all of outer home, inner home, starter and advance starter are clear. If co-located with the starter of the block in rear, slotting must prevent the distant being cleared while the starter is at danger - and this means the distant's lever may be pulled to clear even though the signal is at caution.

The difficulties come when one considers what happens if, for example, circumstances require the inner home to be reset to danger because, say, of an obstruction on the line (or, more likely, because the signaller resets the signals in the wrong order after a train has passed). One source says that the cleared distant locks the stop signals at clear - hard to believe but that would be easy to implement. Alternatively, it could work like slotting and the distant could return to caution when any of the stop signals are set to danger, leaving the signal out of sync with its lever. Again, not difficult to implement but it leads to an obvious further question. If all the stop signals in the block are subsequently cleared and the lever of the distant is still at clear, should the distant clear itself?

The interlocking will enforce the return of each stop signal to danger after the passage of a train before the stop signal in rear can again be cleared (a sort of 'Welwyn control'). If the slotting option applied, the distants could be included in such an arrangement, which would force the signal and lever back into sync - but that doesn't feel like the right approach somehow.

Can anybody help, please?

Andrew

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grovenor-2685
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Re: Distant Signal Locking

Postby grovenor-2685 » Tue Mar 24, 2009 9:05 pm

Andrew,

If as seems to be the case these are semaphore signals worked from a mechanical frame then the situation is very simple.
The distant lever is released by the home, starter et al levers being reverse. When you pull the distant the tappet will push the locking out of the notch on the distant lever and into notches on all the other levers. Now the only lever free to move is the distant. None of the other signal levers can be replaced unless the distant is replaced first.
This is essential for safe operation, if the home could be replaced after clearing the distant the locking on the points etc could be freed with an approaching train still getting a green.
The distant clear must always mean that the home and starter are clear, hence the distant must always be replaced first.
Mechanical interlocking works like that. With colour lights, or even motor operated distants in theory, there is no need of a distant lever as the signal can just clear when all the homes and starters are clear and replace itself when the home is replaced or by occupation of a berth track circuit.

IRSE Green booklets: Distant signal levers must be so interlocked that the signals cannot give a clear indication when any of the relative stop signals are at danger.

Fig. 2. 4 would be released by 5, 6 and 7. When the distant signal is cleared, it indicates to a driver that the road to which it applies is clear through the territory controlled from the signal box. Consequently, 4 lever must not be free to be pulled until all the levers working the running signals ahead have first been pulled. Similarly 25 would be released by 22, 23 and 24.

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andrew jukes
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Re: Distant Signal Locking

Postby andrew jukes » Tue Mar 24, 2009 9:55 pm

Thanks, Keith.

It wasn't the requirement that the distant must not be clear unless all the stop signals are clear that was in doubt, only what happens when you want to reset any stop signal. I was uncomfortable with stop signals being locked 'clear'.

The rationale you give would still be covered if replacing any of the stop signals automatically set the distant to caution - but I can see that would have been difficult with traditional lever-operated locking. So in an emergency, the signaller just has to reach for the distant lever first!

I think the leverless (motor operated) distant comes close to the way I was picturing distants might work. I assume that would also be typical of how a colour light distant in a semaphore area would work?

As you imply, multiple aspect colour lights are easy.

Andrew

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grovenor-2685
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Re: Distant Signal Locking

Postby grovenor-2685 » Tue Mar 24, 2009 10:38 pm

I think the leverless (motor operated) distant comes close to the way I was picturing distants might work. I assume that would also be typical of how a colour light distant in a semaphore area would work?


Note I said theoretically, mostly motor operated signals had their own lever and used normal mechanical locking.I don't know of any leverless motor distants but it is possible so there may have been some. I certainly recollect colour light distants being done that way, I might even be able to turn up a reference. But c/l distants with levers also existed, and were probably the more common. Bit of an LMS specialty though, not in your neck of the woods.
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Re: Distant Signal Locking

Postby andrew jukes » Wed Mar 25, 2009 11:46 am

Turning to 'my neck of the woods', it seems there must have been plenty of locking of the up stop signals by distants. Post-1931, Welwyn had an up outer distant (at Woolmer Green), splitting distants for Digswell jct. between the tunnels, more splitting distants with the Welwyn up home at the north end of the platforms leading finally to the Welwyn up starters which acted as the junction signals at Digswell, at the south end of the viaduct. Pre-1931, there had been a separate box at Digswell, Welwyn's up starter was at the north end of the viaduct (on the viaduct, actually) and Welwyn itself would only have had the Woolmer Green and between the tunnels distants - not splitting then.

I suppose the justification for so many distant signals in series is to give as early notice as possible that the junction has been set, even if the train is already well into the block. So (post-1931) if Digswell jct is set when the train is, say, in Welwyn South tunnel, a cleared distant at Welwyn station is still of some help in keeping a train going at a decent speed.

I shall play a bit fast and loose with the dates and use some pre-1931 features. What is clear though is that the interlocking design not only has to allow a distant to lock the stop signals but allow for several distants in series - I guess three would be a reasonable maximum.

Andrew

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grovenor-2685
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Re: Distant Signal Locking

Postby grovenor-2685 » Wed Mar 25, 2009 6:27 pm

I shall play a bit fast and loose with the dates and use some pre-1931 features. What is clear though is that the interlocking design not only has to allow a distant to lock the stop signals but allow for several distants in series - I guess three would be a reasonable maximum.


Just to keep the terrminology correct, Distant signals are 'Released by' the homes and starters, then the distant reverse 'backlocks' the homes and starters.
The term 'Locks' is when applied to signals is always used when a lever is locked normal, if a lever is locked reverse it is 'Backlocked' ie prevented from being put back. Points on the other hand can be locked normal, locked reverse, or locked both ways.
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andrew jukes
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Re: Distant Signal Locking

Postby andrew jukes » Wed Mar 25, 2009 7:34 pm

Thanks Keith. All clear now (and comprehensively backlocked!)

Regards
Andrew

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Jim Summers
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Re: Distant Signal Locking

Postby Jim Summers » Sun Mar 29, 2009 10:59 am

I've come into this correspondence a bit late; but I always found one bit of the jargon useful when thinking through such things. And that is "Sequential locking". This enforces the pulling off of levers in a logical sequence, and prevents, for example, the Distant being pulled off before Home and Starter (which of course will, of necessity, be already back in the frame).

In passing, I always noted that, compared with the decisive jerk of an upper quadrant above it, a slotted distant always came off in a slower, steadier movement, if it was pulled later than the Starter above it. All to do with the balance weights and the slotting mechanism, but there is a challenge for the "bouncing arms" folk to reproduce.

Jim

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Re: Distant Signal Locking

Postby grovenor-2685 » Sun Mar 29, 2009 9:05 pm

Sequential locking was usually applied between Home and Starter, so that if the home signal was pulled and replaced it could not be pulled again until the starter had also been pulled and replaced, combined with a Line clear release on the starter and a berth track circuit for the home that held the block at 'train on line' this gave the full interlocking, or class C block. For details see this letter: http://www.norgrove.me.uk/signalling/Block-a.pdf
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andrew jukes
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Re: Distant Signal Locking

Postby andrew jukes » Sun Mar 29, 2009 9:41 pm

My computer based equivalent of this, with full track-circuiting, will be for any stop signal to be able to be placed in a special condition in which, once passed by a train, it remains clear but restricts the use of the stop signal in rear. The fundamental reason this is needed on a model is that, unlike colour lights which realistically revert to danger the moment the relevant track circuit is occupied, semaphore stop signals ought to be replaced by the signalman.

In the 'special condition' any stop signal will lock at danger (or place also in the 'special condition' if, improbably, it is clear) the stop signal(s) immediately in rear. When replaced to danger, this lock will be released. This ensures that any stop signal passed by a train has to be replaced to danger before a new signalled route approaching it can be set up, but also allows the replacing at danger to be done by the signalman in his own good time.

Regards

Andrew

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Re: Distant Signal Locking

Postby grovenor-2685 » Sun Mar 29, 2009 10:47 pm

That is what the class C block achieves.
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beast66606
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Re: Distant Signal Locking

Postby beast66606 » Wed May 06, 2009 9:37 pm

When the distant is reversed it holds all the stop signal levers reversed, it was an "amusing" trick of mine to knock one of the home levers out of the catch handle and watch the signalman replace the distant and then have to dive for the home lever to stop it flying back in front of the driver.

Points on the other hand can be locked normal, locked reverse, or locked both ways.


Or rear locked ...
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andrew jukes
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Re: Distant Signal Locking

Postby andrew jukes » Mon Apr 18, 2011 10:09 pm

Another distant signal question, I’m afraid - and not exactly about locking, but it seems to go with with this thread.

Where there are outer and inner distants, is each signal always worked by a separate lever?

I have an example where it’s pretty clear they would have been separate. On the up line approaching Welwyn North, the Woolmer Green up starter immediately north of Welwyn North tunnel had the Welwyn up outer distant on the same post, then between the north and south tunnels was the up inner distant - latterly split to show the routing at Digswell junction, south of Welwyn viaduct.

In simpler cases where there was no need to indicate routing, one lever would seem to offer advantages and few disadvantages. Would that ever have been used?


Incidentally, my education has now progressed to ‘double-slotting’ - which I gather is needed when outer and inner distants are respectively co-located with the previous block’s starter and advance (or advanced?) starter. If I’ve got it right, the outer distant needs to be slotted with both starters so that a driver can’t be presented with clear starter and outer distant, followed by an advance starter at danger with the (slotted) inner distant signal held at caution.

Andrew Jukes

JFS
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Re: Distant Signal Locking

Postby JFS » Tue Apr 26, 2011 9:45 pm

andrew jukes wrote:Where there are outer and inner distants, is each signal always worked by a separate lever?


Hi Andrew,

it is always wrong to generalise and and every rule has its exception, but I will stick my neck out and say I can think of no circumstances where a single lever worked more than one arm on separate posts (ie I am excepting co-acting arms). In the case of inner and outer distants, there was always a reason for their provision and the Local Operating instructions would have regulated the circumstances under which they were to be worked.

As an example, the Western, in densely signalled areas, had a peculiar arrangement allowing trains to be accepted "under Reg. 4A" (Line clear to clearing point only - belled as 2-2-2) Now by definition, if the line is clear to the Clearing Point, then the train should be accepted! The purpose of all this was to tell the box in rear to maintain the distant (often an inner distant) at Caution. This saved a lot of slotting of distants but was a bit of a cheat on the part of the Western. On Other Railways, the same eventuality was covered by slotting the distants - thus, the inner distant of one box was the outer distant of the next. Thus both boxes had to reverse their levers before the arm would clear (this is what I understand as a double slot).

This, I think would be a very common arrangement, and probably the most common reason for the provision of inner and outer distants. SO, in that circumstance, why NOT work the two distants from one lever? Well, I think the mechanics of it (two adjusters, different weights of slot / arm etc) would make it pretty unreliable to work. BUT I expect someone will come up with a contrary example:-)

The only way to check these things out would be to check the box diagrams, (which always show when an arm is slotted by another box) locking charts and local instructions for all the boxes in the area. Local instructions in particular are not easily come by though the SRS collections are always the best place to start - join up, and make friends with the Corresponding Member for you area!

Best Wishes,

Howard.

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grovenor-2685
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Re: Distant Signal Locking

Postby grovenor-2685 » Wed Apr 27, 2011 8:35 pm

Where there are outer and inner distants, is each signal always worked by a separate lever?


This is probably an issue of definition. Inner and outer distants as seperate signals are rather uncommon. I have only seen them on the GN. (Of course pertinent to your interest). Good examples can be seen here
http://www.signalbox.org/diagrams.php?id=440
i would suspect that outer distants were added to cater for the streamliners, you may know better, although that does not explain why they are provided for the slow lines as well. I would assume in this case that 1 is released by 2 and 2 in turn released by 3 and 4. similarly for the other 3 sets.

More commonly multiple distant arms are worked by the one lever and exist only as the boxes are so close together that there are several signals at the box in rear within the braking distance of the home, a distant arm may then be provided and slotted with each such signal. An example here
http://www.signalbox.org/diagrams.php?id=361
Note that lever 3 operates all the distant arms but as you go back each arm has an extra slot, so the distant at 458 yds is just slotted with the signal whose post it shares, advanced starter perhaps, the arm at 738 yds is slotted by the signal on the same post and the advanced starter as well then the final distant at 1200 yards is triple slotted by all 3 signals.
I would think both the distant and the advance starter in rear each with 3 slots would help the signalmen build up their arm muscles.

but I will stick my neck out and say I can think of no circumstances where a single lever worked more than one arm on separate posts (ie I am excepting co-acting arms).

Well with regard to distants see second example above. Also sometimes levers were saved by selecting the correct arm by the point detection, although then it was only one signal at a time, and usually shunt or similar low speed cases.

The purpose of all this was to tell the box in rear to maintain the distant (often an inner distant) at Caution. This saved a lot of slotting of distants but was a bit of a cheat on the part of the Western. On Other Railways, the same eventuality was covered by slotting the distants - thus, the inner distant of one box was the outer distant of the next. Thus both boxes had to reverse their levers before the arm would clear (this is what I understand as a double slot).

A double slot is usually just two stop signals slotting the same distant. What you are suggesting by your Western cheat i assume is double block working being used to allow faster trains than the signalling is designed for. A practice used elsewhere as well I believe but unlikely to get through a hazard analysis these days.

This, I think would be a very common arrangement, and probably the most common reason for the provision of inner and outer distants.
My feeling is that the arrangement you describe would be quite rare given that having inner and outer distants is not common to start with. Can you cite any examples?

regards
Keith

andrew jukes
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Re: Distant Signal Locking

Postby andrew jukes » Fri Apr 29, 2011 8:25 pm

Thanks, Howard and Keith. Both interesting and helpful.

I have applied for SRS membership - thought about it before but you have overcome the inertia.

The Belle Isle Up diagram is something I really needed, quite apart from the multiple distants question. I suspect multiple distants here as at Welwyn are partly a result of sighting and location problems caused by tunnels.

At Welwyn, an up mineral joining the two-track section at Woolmer Green would be more likely to keep moving if given a second chance between the tunnels of knowing it was clear to Digswell (at the south end of the viaduct). The extreme spread of speeds (20 v 80mph?) would make it quite likely the block would become clear after the mineral has passed the outer distant.

At Belle Isle, you say - “I would assume in this case that 1 is released by 2 and 2 in turn released by 3 and 4. similarly for the other 3 sets.” Does that mean that 2 is also backlocked by 1?

The Wednesbury distants are surprising. As the stop signals are unnumbered, I assume they all ‘belong’ to the box in rear - in which case I would expect the distant at 458yds to be slotted with its co-located stop signal, with the ones in rear being double- and triple- slotted. The diagram (if I understand the convention) seems to show them all with one slot less than this.

Incidentally, is there anywhere I can find the diagram conventions (to the extent they were standardised)?

Thanks again.

Andrew

ps I gather Network Rail are contemplating fiddling with the signalling at Welwyn to give reduced headways - some problems are always with us!

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grovenor-2685
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Re: Distant Signal Locking

Postby grovenor-2685 » Fri Apr 29, 2011 10:12 pm

Does that mean that 2 is also backlocked by 1?
Certainly does. Effectively1 can only be cleared if all signals at the block post are clear, then 1 has to be replaced before the others.

I would expect the distant at 458yds to be slotted with its co-located stop signal, with the ones in rear being double- and triple- slotted.

I think that is what I tried to explain, so far as the diagram goes slotting between home and distant on the same post is a given so only extra slots are shown by the slot symbol. These symbols were probably in the old BS 365 IIRC but that was not always followed and lots of diagrams pre-date it. I will have to get hold of a copy to see what it actually covers.
Regards
Keith

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beast66606
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Re: Distant Signal Locking

Postby beast66606 » Fri Jun 03, 2011 7:22 pm

JFS wrote:it is always wrong to generalise and and every rule has its exception, but I will stick my neck out and say I can think of no circumstances where a single lever worked more than one arm on separate posts (ie I am excepting co-acting arms). In the case of inner and outer distants, there was always a reason for their provision and the Local Operating instructions would have regulated the circumstances under which they were to be worked.


Quite common, several boxes on my local line (Chester - Birkenhead Woodside) for example had multiple distant signals worked by one lever.

JFS wrote:As an example, the Western, in densely signalled areas, had a peculiar arrangement allowing trains to be accepted "under Reg. 4A" (Line clear to clearing point only - belled as 2-2-2) Now by definition, if the line is clear to the Clearing Point, then the train should be accepted! The purpose of all this was to tell the box in rear to maintain the distant (often an inner distant) at Caution. This saved a lot of slotting of distants but was a bit of a cheat on the part of the Western.


Maybe they should have used it at Wrexham ...

JFS wrote:On Other Railways, the same eventuality was covered by slotting the distants - thus, the inner distant of one box was the outer distant of the next. Thus both boxes had to reverse their levers before the arm would clear (this is what I understand as a double slot).


No its just a slot, and other things could be used such as face discs or indicators, again I can think of several local examples and even own a distant indicator for Blackpool Street Down Distant, which would have been situated in Green Lane Junction box.


JFS wrote:This, I think would be a very common arrangement, and probably the most common reason for the provision of inner and outer distants. SO, in that circumstance, why NOT work the two distants from one lever? Well, I think the mechanics of it (two adjusters, different weights of slot / arm etc) would make it pretty unreliable to work. BUT I expect someone will come up with a contrary example:-)


plural.

JFS wrote:The only way to check these things out would be to check the box diagrams, (which always show when an arm is slotted by another box) locking charts and local instructions for all the boxes in the area. Local instructions in particular are not easily come by though the SRS collections are always the best place to start - join up, and make friends with the Corresponding Member for you area!


Which used to be me for the Birkenhead joint.
DAS
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All opinions are mine and mine alone
http://www.wirralfinescale.com
http://www.johnskipsey.fotopic.net


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