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| Why is the Clearing Point 1/4 Mile? 12/11/2014 at 15:55 #65576 | |
BoxBoyKit
166 posts |
Hi everyone, I was just wondering if anyone would be able to give any explanation as to why it was decided the Clearing Point should be 1/4 mile, if indeed there is any genuine reason. I'm planning a potential talk on the basics of signalling, and I'd much prefer to say "This is why", rather than, "It just is." Many thanks in advance. Log in to reply |
| Why is the Clearing Point 1/4 Mile? 12/11/2014 at 16:25 #65577 | |
KymriskaDraken
963 posts |
Now that is an interesting question. My money is on "it just is" as so many things are. Kev Log in to reply |
| Why is the Clearing Point 1/4 Mile? 12/11/2014 at 16:41 #65578 | |
CTCThiago
232 posts |
" said:Hi everyone,Hello mate, You can check this link: http://en.wikipedia.org/wiki/Absolute_block_signalling#Clearing_point Preview: Clearing point: The clearing point is a point usually ¼ mile (440 yards) in advance of a signal box's home signal. If, for some reason, the train does not immediately proceed beyond B and remains within the clearing point, the signalman cannot send "Train out of Section" to the signalbox in rear until the clearing point is clear. This is to give some safety margin in the event of a following train misjudging its braking to a stand at the home signal. Similarly, if the clearing point is obstructed for some other reason, such as shunting, a train cannot be accepted. The clearing point is usually defined by some obvious marker - at some wayside locations, it may simply be the starting signal in the same direction, whilst at others it may be another signal, or require a track circuit to be clear. Additionally, no two clearing points may overlap - for example, at junctions with conflicting movements, the points should be set so that, should each train fail to stop, they would not collide. Similarly, a full ½ mile must be given for trains approaching each other on the same track - ¼ mile for each train. A further rule regarding clearing points is that they cannot be modified once a train has been accepted, and that they must be locked for passenger train movements. Or this one here too: http://www.signalbox.org/block/absolute02.htm Preview: The clearing point is a quarter of a mile ahead of the first stop signal controlled by a box, and is a safety zone - an area that must not be occupied or fouled by another train. It allows for the approaching train being unable to stop at the signal, perhaps through slippery rails or weak brakes. Fonts are: Wikipedia and TheSignalBox. Regards, Thiago. Log in to reply |
| Why is the Clearing Point 1/4 Mile? 12/11/2014 at 19:40 #65580 | |
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Finger
220 posts |
" said:Now that is an interesting question. My money is on "it just is" as so many things are. Given how different these are across different railways in Europe, even in GB between AB and TCB, I'm inclined to agree that "it just is". The only case where something like overlap length is defined by something exact is when it is the breaking distance from the permitted speed before the signal and the train is running with an ATP system designed to act only on passing of the stop signal (but controlling the speed before the signal). This allows the driver to run as fast as he pleases before the stop signal and brake sharply as opposed to crawling like a snail. Log in to reply |
| Why is the Clearing Point 1/4 Mile? 12/11/2014 at 22:17 #65583 | |
jc92
3630 posts |
" said:" said:Given that the majority of clearing points are measured to the nearest identifiable location, eg to the inner home/starter, or "to that lineside hut over there" etc, and given that that it is altered based on linespeed, gradient, local conditions etc, I would surmise 440Yds is the default figure given as a benchmark, but as with so much British signalling practice, the exception is more common than the rule.Now that is an interesting question. My money is on "it just is" as so many things are. "We don't stop camborne wednesdays" Log in to reply |
| Why is the Clearing Point 1/4 Mile? 13/11/2014 at 09:47 #65585 | |
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kbarber
1712 posts |
" said:" said:" said:Given that the majority of clearing points are measured to the nearest identifiable location, eg to the inner home/starter, or "to that lineside hut over there" etc, and given that that it is altered based on linespeed, gradient, local conditions etc, I would surmise 440Yds is the default figure given as a benchmark, but as with so much British signalling practice, the exception is more common than the rule.Now that is an interesting question. My money is on "it just is" as so many things are. I think the answer is very much 'It just is'. The concept of a clearing point arose after the Abbots Ripton collision of 1876. The Inspecting Officer's report referred to the lack of a distance beyond the home signal that was required clear before a second train could approach, with the memorable phrase that the safety margin might be 'no greater than the thickness of a signal post'. Interestingly, that comment led to no recommendation. The 1/4 mile requirement came at some later stage in the Board of Trade requirements for signalling (I don't have any reference to hand that would say when) but was probably derived from that comment. I think the distance was one that was considered reasonable in the circumstances but with little else in the way of a rationale. 1/4 mile was very much a nominal distance and, in any case, only applied on passenger lines. (It was normal, even if absolute (rather than permissive) block was being worked, to find the clearing point on goods lines was the home signal - i.e. 0 yards.) Where an outer home was provided more than 440 yards outside the inner home, the clearing point became the inner home rather than the 1/4 mile mark (of course where the distance was less than 440yd the 1/4 mile rule was followed). Sometimes the terminating point of specific track circuits would be identified as the clearing point (the down lines at St Albans South were treated thus). And sometimes local instructions would permit a reduced clearing point (I believe Exeter Middle could accept - on one of the up lines at least - with the line clear to the mid point of the signalbox). And that's before we start getting into the delights of short sections, fog working and Regulation 5... Log in to reply |
| Why is the Clearing Point 1/4 Mile? 13/11/2014 at 13:37 #65587 | |
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Finger
220 posts |
" said:Given that the majority of clearing points are measured to the nearest identifiable location, eg to the inner home/starter, or "to that lineside hut over there" etc, and given that that it is altered based on linespeed, gradient, local conditions etc, I would surmise 440Yds is the default figure given as a benchmark ... I think you might be committing yourself to a fallacy of imperfection, like saying that because you can't draw a perfectly round circle, circles are not round. Also, introducing a red herring, because the question was where does the 400m normative value come from, not how precisely does it adhere to BR/NR practice. Log in to reply |
| Why is the Clearing Point 1/4 Mile? 13/11/2014 at 21:05 #65590 | |
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BoxBoyKit
166 posts |
Many thanks for the replies. Certainly looks like "it just is" will have to do as the definitive answer!
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| Why is the Clearing Point 1/4 Mile? 14/11/2014 at 17:38 #65595 | |
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slatteryc
248 posts |
Is it a stupid question to ask what the difference between the Clearing Point and the overlap is ?
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| Why is the Clearing Point 1/4 Mile? 14/11/2014 at 18:05 #65598 | |
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AndyG
1835 posts |
I tend to think of a CP is related to AB working, relating to accepting a train as far as your home signal, whereas o/lap is for (broadly) MAS and tends to be for every signal on passenger lines.
I can only help one person a day. Today's not your day. Tomorrow doesn't look too good either. Log in to reply |
| Why is the Clearing Point 1/4 Mile? 15/11/2014 at 09:55 #65603 | |
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kbarber
1712 posts |
" said:I tend to think of a CP is related to AB working, relating to accepting a train as far as your home signal, whereas o/lap is for (broadly) MAS and tends to be for every signal on passenger lines. That would be my interpretation too. Of course it's not quite that simple. (It never is where signalling is concerned!) There are some Absolute Block areas where signals do have overlaps, whether they're referred to as such or not. This would usually be where a box has ended up controlling a number of successive colour light signals. So Finchley Road acquired signals with overlaps after the frame was renewed (and West Hampstead abolished) in the spring of 1978. Of course this was part of the stageworks for the present West Hampstead PSB so it's not really surprising the new signals worked that way. (It also meant Finchley was authorised to clear back to Cricklewood without seeing tail lamps, once a specified series of track circuits on each line had been occupied and cleared; in this case the clearing point was in fact the end of what would become an overlap once WH took over.) Likewise Biggleswade (formerly Biggleswade South) came to control a series of 3-aspect colour lights on the up fast after B'wade North and Langford Bridge were abolished (early or mid 1960s I think). Working with Sandy was conventional AB with a 2-aspect up fast distant well over a mile out (somewhere near the sewage works, for anyone who knows the area). The Biggleswade home was a good 1/4 mile north of the former North box, just north of Crab Lane; the former North Box homes had been just north of High Street, about 200yd further south. There was then a starter well south of the A1 overbridge, not far in rear of Holme Green Level Crossing (long since closed). Finally there was an IB Home a good distance to the south (the picture I have of the diagram isn't quite readable but I'm sure it's in excess of 3000 yards from the box), which also acted as Arlesey distant. The home has an overlap track extending all the way through the station and almost to the box, thus enforcing a clearing point that would in practice have been around 1/2 mile long (I imagine it was in the block controls so the block would be held at Train On Line until that track cleared). The starter and the IB had overlap track circuits, interestingly 300 yards in both cases. The working to Arlesey was AB on the up fast, with no TC between the terminating point of the IB overlap and Arlesey; interestingly the working on the down fast was TCB from Arlesey with AB and an IB towards Sandy and again the overlaps seem to have been 300yd. The upshot was that, to a driver, it was as if there was an island of modern multi-aspect signalling (indeed this kind of setup was sometimes referred to, I believe, as "drivers' mas"). There were several stretches of this kind on the GN and, had the Kings X resignalling not happened, I suspect the S&T would slowly have converted the whole line thus, just as the Civil Engineers were slowly fettling the whole line to take 100mph running and give the expensively-rebuilt WCML a run for its money! [/partisan] :-) Log in to reply The following users said thank you: AndyG, CTCThiago |
| Why is the Clearing Point 1/4 Mile? 15/11/2014 at 11:12 #65604 | |
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Danny252
1461 posts |
" said:There are some Absolute Block areas where signals do have overlaps, whether they're referred to as such or not. This would usually be where a box has ended up controlling a number of successive colour light signals.I can even think of a number of cases where semaphore signals within station limits effectively had (or have!) overlaps - e.g. requiring a set of points in front of a signal to be in a certain position, or a TC to be clear, before the signal in rear could be cleared. Log in to reply |
| Why is the Clearing Point 1/4 Mile? 15/11/2014 at 17:12 #65609 | |
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kbarber
1712 posts |
" said:" said:Should've come back on this before.Hi everyone,<snip> Some companies insisted on the clearing point being unmodifiable once a train had been accepted, until it had passed or come to a stand. But others were more easygoing. (At least I assume this must go back to individual company practices, I can't imagine any other way it came about.) On the Midland at Finchley Road the clearing point on the down was altered - often with a train already in section, not just accepted - many times a day. In fact it would have been pretty much impossible to operate the timetable if alteration of the clearing point was banned. If you have the West Hampstead sim, you need to imagine the fast lines being goods lines and continuing as a double track into the up & down Hendon lines. Then the passsenger lines used the other tunnel and split, about 150m north of the mouth of Belsize Tunnel, into fast and local lines (nowadays fast & slow). Empty cars and light engines to Cricklewood, as well as slower passenger trains running to the down local, had to cross the up fast on the level in a conventional double junction (albeit with switch diamonds by the 1970s). There were something like 20 trains passing in each direction during the peak. The next box to the south was Carlton Road Junction but the local instructions required Carlton Road to offer trains pretty much as soon as they left St Pancras. There would have been no possibility whatsoever of setting a clearing point towards the down local without blocking a train on the up fast. But waiting until the down road had come to a stand before swinging the points and pulling off would have caused significant delays to following services. So the junction was swung after the up road had passed, often with the headlights of the approaching down road clearly visible in the tunnel. I should emphasise there were no special instructions modifying the clearing point; this was the accepted method of doing the job. However the preamble paragraphs to the Absolute Block regulations (I'm looking at the 1972 issue) seem to legitimise the practice; in the section titled 'Working of Fixed Signals at Diverging Points', the 4th paragraph reads "When a signalman at a junction or other diverging point is not in a position to set up the route required for a train until the train is closely approaching the stop signal at such diverging point, he must, as far as practicable, before he sets the points and clears the signal for the required route, satisfy himself that, having regard to the position and speed of the train, it is safe for him to do so." As I said, nothing is ever that simple where signalling is concerned (and I should have added that it was even less so where AB and mechanical signalling were in use). Log in to reply |
| Why is the Clearing Point 1/4 Mile? 15/11/2014 at 18:36 #65612 | |
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Puro
18 posts |
The clearing point is an overlap, as in buffer no train track! The overlap distances in Portugal will vary from a one train at a time into a single track station rule in mechanical signalling to 150 meters for trains & 50 meters for shunt & proceed on sight movements Log in to reply |