AB regulations: A question about clearing points

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AB regulations: A question about clearing points 06/12/2019 at 22:41 #122124
TUT
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In the modern Rule Book, regulation 3.4.1 of Module TS3 Absolute block regulations states:

Quote:
3.4.1 Before allowing a train to approach

Before you allow a train to approach from signal box A, you must make sure that all the following conditions apply.

• The line, or at a facing junction the line for which the facing points are set, is clear up to and including the clearing point.
• All points within the clearing point have been set for the safety of the approaching train.
• No conflicting movement has been authorised that will cross or foul the line within the clearing point.
• No train has been accepted from another direction that requires a portion of the same line within the clearing point for acceptance.
Now I can readily appreciate that the Rule Book is written to be as general and all-encompassing as possible, but I do have a question about the last two bullet points.

"No conflicting movement has been authorised that will cross or foul the line within the clearing point"

So perhaps we have a connection from sidings, or a crossover, or a converging junction within the clearing point, and this bullet point is telling us that we cannot accept a train if we've already authorised a conflicting movement. North East Scotland provides a perfect example. At Blackford, you cannot accept a train from Greenloaning if you've cleared BK5 for a movement from the Down Sidings, BK15 for a movement from the Up Sidings, or BK16 for a movement to cross from the Up Perth to the Down Perth, as these movements will cross or foul the line within the clearing point.

But here's my question. All of those moves require points within the clearing point reverse, e.g. to clear BK5 requires BK6 points reverse.

But the second bullet point already specifies "all points within the clearing point have been set for the safety of the approaching train".

And it's a similar consideration for the last bullet point. How could I accept a train from another direction that requires a portion of the same line within the clearing point for acceptance and simultaneously have all points within the clearing point set for the safety of both approaching trains.

I mean, it seems to me the rigid application of bullet point 2 would cover bullet points 3 and 4 as well!

So are bullet points 3 and 4 redundant (although I'm not saying they're not worthwhile anyway), or do they cover any real edge cases. I suppose I can picture a wrong-direction shunting movement or some kind of movement over a fixed crossing that might be covered by point 3 and not point 2, but a concrete example would be nice

Last edited: 06/12/2019 at 22:43 by TUT
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AB regulations: A question about clearing points 07/12/2019 at 02:32 #122125
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probably a case of belt and braces safety however this would also cover diamond crossovers within the clearing point for instance where two clearing points conflict without the lay of points being an issue.

example attached. the red train is going to shunt into siding A. the lay of the points is irrelevant to any train going from C to B so a train could be accepted with them reversed, but couldn't be accepted while the train is making a movement over the crossover (assuming the clearing point is beyond the crossover).


this kind of layout was incredibly common on the midland railway to avoid facing points into sidings although I'd assume other companies also had similar layouts.

points 3 and 4 would also cover movements into the clearing point where for instance a LOS is provided (blocking back is only required if a movement will come to a stand in the clearing point, not if it is only passing through it).

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AB regulations: A question about clearing points 07/12/2019 at 02:50 #122126
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You're a star, thank you.

The bullet points all make sense in words - they basically explicitly prohibit every which way of accepting a train with the clearing point fouled, obstructed, or about to be fouled or obstructed.

But it's just so much more meaningful, to me at least, when you have a picture in your head to help you understand the words more fully.

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AB regulations: A question about clearing points 07/12/2019 at 09:28 #122128
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A conventional double junction might also result in a clearing point fouled by a movement (or indeed a clearing point) to/from the opposite line through the diamond crossing. Finchley Road might have been a case in point if it hadn't been for having switch diamonds, rather than a fixed set, in the up fast.
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AB regulations: A question about clearing points 07/12/2019 at 14:29 #122129
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Quote:

• The line, or at a facing junction the line for which the facing points are set, is clear up to and including the clearing point.
• All points within the clearing point have been set for the safety of the approaching train.
• No conflicting movement has been authorised that will cross or foul the line within the clearing point.
• No train has been accepted from another direction that requires a portion of the same line within the clearing point for acceptance.
I can think of a location where 2) is satisfied when a crossover is reverse, and you can legitimately accept a train travelling towards you on the up, or on the down.

However if you accepted both at the same time with the crossover reverse you have broken 4) as the trains are sharing a good chunk of the clearing point.

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AB regulations: A question about clearing points 07/12/2019 at 18:14 #122131
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Thanks very much both of you.

I'm glad I asked, because it helps me be surer of my interpretation.

headshot119 in post 122129 said:
Quote:

• The line, or at a facing junction the line for which the facing points are set, is clear up to and including the clearing point.
• All points within the clearing point have been set for the safety of the approaching train.
• No conflicting movement has been authorised that will cross or foul the line within the clearing point.
• No train has been accepted from another direction that requires a portion of the same line within the clearing point for acceptance.
I can think of a location where 2) is satisfied when a crossover is reverse, and you can legitimately accept a train travelling towards you on the up, or on the down.

However if you accepted both at the same time with the crossover reverse you have broken 4) as the trains are sharing a good chunk of the clearing point.
Interesting! Are you able to share where abouts?

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AB regulations: A question about clearing points 07/12/2019 at 18:19 #122132
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Workington Main No 3 Digram photo. 39s being the crossover I'm referring to.
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AB regulations: A question about clearing points 08/12/2019 at 10:04 #122136
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Here's another example I thought of.

in the drawing below, I can't accept a train from A and C at the same time, even with the points reversed as the clearing point for both trains will overlap. Nowadays most locations like this will have an outer acceptance home (which is the case on the Harrogate Loop boxes).

We have this arrangement at Darley Dale, but have a dispensation from HMRC that the clearing point from C is signal post thick given the 15mph speed limit and uphill gradient which allows two trains to approach at the same time.


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AB regulations: A question about clearing points 08/12/2019 at 10:09 #122137
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headshot119 in post 122129 said:
Quote:

• The line, or at a facing junction the line for which the facing points are set, is clear up to and including the clearing point.
• All points within the clearing point have been set for the safety of the approaching train.
• No conflicting movement has been authorised that will cross or foul the line within the clearing point.
• No train has been accepted from another direction that requires a portion of the same line within the clearing point for acceptance.
I can think of a location where 2) is satisfied when a crossover is reverse, and you can legitimately accept a train travelling towards you on the up, or on the down.

However if you accepted both at the same time with the crossover reverse you have broken 4) as the trains are sharing a good chunk of the clearing point.
Crewe Junction aswell. If they accept a train from Crewe Bank and have the clearing point set into P4, they can't accept a train from SBJ into P4 even though all points are set safely within both clearing points.

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AB regulations: A question about clearing points 08/12/2019 at 10:26 #122138
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jc92 in post 122137 said:

Crewe Junction aswell. If they accept a train from Crewe Bank and have the clearing point set into P4, they can't accept a train from SBJ into P4 even though all points are set safely within both clearing points.
There's probably a lot of examples around the country, especially around stations with multiple signalboxes.

For some reason Workington Main No3 was the one that came to my mind first.

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AB regulations: A question about clearing points 08/12/2019 at 18:24 #122147
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I hope you won't think I'm taking liberties, but if I might put my understanding to the test?

"The line, or at a facing junction the line for which the facing points are set, is clear up to and including the clearing point."

So with regard to "or at a facing junction the line for which the facing points are set" we could use this old layout at Morebath Junction as a concrete example: https://www.s-r-s.org.uk/html/gwe/S772.htm

So perhaps we have a train awaiting acceptance on the Up Main at the section signal (21 I think). We could accept another up train from Dulverton if the junction points are set for the branch and the line is clear 440 yards beyond the home signal.

"All points within the clearing point have been set for the safety of the approaching train."

So at Ashwater as it was then: https://www.s-r-s.org.uk/html/srq/S3564.htm we would need 5 points reverse and 6 points (trailing crossover) normal before accepting an up train from Tower Hill.

"No conflicting movement has been authorised that will cross or foul the line within the clearing point."

Assuming for the sake of argument (though it seems likely) the crossover was in the clearing point, we could use the example of Venn Cross https://www.s-r-s.org.uk/html/gwe/S770.htm We cannot accept a down train from Wiveliscombe with a route set for a train to enter or leave the siding over that long crossover which I believe was number 17 (discs seem to be 16 and 18).

"No train has been accepted from another direction that requires a portion of the same line within the clearing point for acceptance."

Finally we could use Leigh Bridge here as an example: https://www.s-r-s.org.uk/html/gwe/S757.htm

So let's say we've got a train on the Up Main at number 9 signal wanting to go to Crowcombe. However, we've accepted a down train from Crowcombe so that's gonna require a 440 yd clearance distance over the Up and Down Main. Now let's say we're offered another train up train from Stogumber. Now we could have accepted that train with 7 points reverse and a 440 yard clearance distance over the Up and Down Main. But because we've already accepted the down train from Crowcombe, by this bullet point we cannot accept the second Up train. (Let's just assume the Up and Down is short enough and the home signals are positioned so that the clearing points would overlap)

Have I got these rules down?

(Yes the choice of single lines is deliberate, I think one of you may have noticed I'm tried to kill two birds (TS3 and TS4) with one stone.)

Last edited: 08/12/2019 at 18:28 by TUT
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AB regulations: A question about clearing points 08/12/2019 at 22:25 #122157
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1) Yes, (You could also accept a train with the points set for the up main assuming that the line was clear 440yards beyond the home.)

2) Yes (But there's no reason you couldn't accept a train with 5 normal (Assuming the FPL can be locked with it normal, if you where shunting a train over 6 reverse), then swing the points when the train was at a stand at the home.)

3) Yes

4) Yes

On a more general note if you are looking at both TS3, and TS4, it's important to note the subtle difference in the location of the clearing point under TS4. It may only go as far as the loop exit signal if the distance is less than normally required, rather than the full distance based on the type of distant signal.

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AB regulations: A question about clearing points 08/12/2019 at 22:39 #122159
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headshot119 in post 122157 said:
2) Yes (But there's no reason you couldn't accept a train with 5 normal (Assuming the FPL can be locked with it normal, if you where shunting a train over 6 reverse), then swing the points when the train was at a stand at the home.)
But, if you did that, you would almost certainly not be able to accept a down train from Halwill by the last bullet point?

Thank you for your help! I'm very grateful, because this part of TS4 seems to have been lifted verbatim from TS3 (or the regulation was composed for both modules simultaneously, perhaps). The older regulations for lines worked by electric token from the days of the big 4 and into BR days do not read like this at all. I'm used to thinking of electric token block in terms of passing loops and this fourth bullet point in particular seemed like a totally moot point to me, because as you say, the clearing point is usually the loop exit signal, so ordinarily the system is designed to allow you to accept trains from both directions simultaneously and at first I couldn't picture a scenario where that would come up.

It's very nice to get confirmation from somebody more experienced in these matters than me, because I was trained on TCB, so when I read TS4 light bulbs don't necessarily light up like they should and I haven't really got a trainer to ask.

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AB regulations: A question about clearing points 08/12/2019 at 22:56 #122160
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Quote:
But, if you did that, you would almost certainly not be able to accept a down train from Halwill by the last bullet point?
Correct.

Two locations that immediately spring to mind to me for some reason where the clearing point is not the loop exit signal are Bransty, and Sellafield, neither of which are crossing loops either. On the other hand neither involve points in the clearing point.

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AB regulations: A question about clearing points 10/12/2019 at 14:40 #122194
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jc92 in post 122125 said:
probably a case of belt and braces safety however this would also cover diamond crossovers within the clearing point for instance where two clearing points conflict without the lay of points being an issue.

example attached. the red train is going to shunt into siding A. the lay of the points is irrelevant to any train going from C to B so a train could be accepted with them reversed, but couldn't be accepted while the train is making a movement over the crossover (assuming the clearing point is beyond the crossover).


this kind of layout was incredibly common on the midland railway to avoid facing points into sidings although I'd assume other companies also had similar layouts.

points 3 and 4 would also cover movements into the clearing point where for instance a LOS is provided (blocking back is only required if a movement will come to a stand in the clearing point, not if it is only passing through it).
Sorry for a hijack, but what was the "common" layout if trains from C also needed to access the siding A? Would there be a trailing crossover after the diamond somewhere for the train to reverse back to where the example train is now, or would the diamond be replaced with a single slip to allow access via the "top" line? (seem to recall slips not being popular outside termini -- perhaps low speed limits or something?), or would there be trailing access from the far end of the sidings via some kind of arrival line?

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AB regulations: A question about clearing points 10/12/2019 at 14:45 #122195
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I expect there would be a separate crossover, which may or may not be immediately adjacent to the siding access. Such crossovers were commonly provided at stations which had their own signalbox, whether or not there was also a goods siding on either side, and this could be used to shunt "the local" Parliamentary service out of the way of more important trains (ie. all of them).
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AB regulations: A question about clearing points 10/12/2019 at 14:53 #122196
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Sorabain said:
Sorry for a hijack, but what was the "common" layout if trains from C also needed to access the siding A? Would there be a trailing crossover after the diamond somewhere for the train to reverse back to where the example train is now, or would the diamond be replaced with a single slip to allow access via the "top" line? (seem to recall slips not being popular outside termini -- perhaps low speed limits or something?), or would there be trailing access from the far end of the sidings via some kind of arrival line?
Neither option would be unusual. It's likely that a trailing crossover would be incorporated into the diamond to form a single slip; these were very common on mechanical layouts (or at least the ones in my L&Y signal diagram books!). It's a marked contrast to today, where, as you say, it's quite rare to find slips where any of the lines involved have a higher speed limit.

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