Phantom Overlaps

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Phantom Overlaps 13/05/2017 at 19:11 #95070
Giantray
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332 posts 		Can someone with a technical background explain Phantom Overlaps please. I understand why they are used, but I am having trouble understanding how the interlocking knows the overlap is clear to the POL and not the standard overlap which could be occupied?
Retired Professional Railwayman (1981-2023); Pway & S&T (1981-88); Former Signalman/Signaller/ Signalling Trainer (1989-2023) [AB, TCB, Mechanical, NX, WestCad, Hitachi SARS]; Railway Historian (esp.SER, LCDR); Member of The Permanent Way Institution..
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Phantom Overlaps 13/05/2017 at 19:22 #95071
JamesN
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1577 posts 		Giantray in post 95070 said:
Can someone with a technical background explain Phantom Overlaps please. I understand why they are used, but I am having trouble understanding how the interlocking knows the overlap is clear to the POL and not the standard overlap which could be occupied?
They tend to be found where trailing points are within the POL. Conditions on the signal would be along the lines of Txxx clear OR points Pyyy reverse/normal as appropriate.
Last edited: 13/05/2017 at 19:23 by JamesN
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Phantom Overlaps 13/05/2017 at 20:25 #95074
GeoffM
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Online 		JamesN in post 95071 said:
Giantray in post 95070 said:
Can someone with a technical background explain Phantom Overlaps please. I understand why they are used, but I am having trouble understanding how the interlocking knows the overlap is clear to the POL and not the standard overlap which could be occupied?
They tend to be found where trailing points are within the POL. Conditions on the signal would be along the lines of Txxx clear OR points Pyyy reverse/normal as appropriate.
I don't think it matters if it's facing points or trailing points beyond the POL. The track circuit is longer than it needs to be so the POL is "at" the correct distance usually, with the points beyond (there may still be locked points before the POL). Since the points are beyond the overlap, they don't need to be in any specific position so are neither called nor detected. However, the TC still needs to be clear. I suppose you could consider the POL marker as the distance beyond which no more locking/checking is in effect.
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Last edited: 13/05/2017 at 20:41 by GeoffM
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The following user said thank you: Giantray
Phantom Overlaps 13/05/2017 at 20:40 #95075
Hawk777
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386 posts 		GeoffM in post 95074 said:
The ones I've seen tend to be facing points in the overlap. The track circuit is longer than it needs to be so the POL is "at" the correct distance usually, with the facing points beyond. Since the points are beyond the overlap, they don't need to be in any specific position so are neither called nor detected. However, the TC still needs to be clear. I suppose you could consider the POL marker as the distance beyond which no more locking/checking is in effect.
Jumping in here a bit, but this is quite interesting to me. What would be the advantage of a phantom overlap ending in rear of the points, in contrast to a swingable overlap including the points? In both cases you would be able to move the points with a route set to the protecting signal.
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Phantom Overlaps 13/05/2017 at 21:01 #95076
Stephen Fulcher
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2028 posts
Online 		Saves money. One track circuit and associated Interlocking instead of two.
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Phantom Overlaps 16/05/2017 at 13:45 #95132
Giantray
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332 posts 		GeoffM in post 95074 said:


I don't think it matters if it's facing points or trailing points beyond the POL. The track circuit is longer than it needs to be so the POL is "at" the correct distance usually, with the points beyond (there may still be locked points before the POL). Since the points are beyond the overlap, they don't need to be in any specific position so are neither called nor detected. However, the TC still needs to be clear. I suppose you could consider the POL marker as the distance beyond which no more locking/checking is in effect.
Thank you Geoff, I knew someone would explain it simply. Explains why they are used and are going to be used in the London Bridge Platforms 6,7,8 & 9. If the full overlap needed to be locked by points, certain moves would not be possible on adjacent lines, but by allowing certain points to be free of the overlap, this would free up other moves.
Retired Professional Railwayman (1981-2023); Pway & S&T (1981-88); Former Signalman/Signaller/ Signalling Trainer (1989-2023) [AB, TCB, Mechanical, NX, WestCad, Hitachi SARS]; Railway Historian (esp.SER, LCDR); Member of The Permanent Way Institution..
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Phantom Overlaps 16/05/2017 at 13:48 #95133
Giantray
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332 posts 		Stephen Fulcher in post 95076 said:
Saves money. One track circuit and associated Interlocking instead of two.
Not entirely true. It maybe that by providing the additional track circuits, they may not be long enough to be allowed as there is a minimum length a track circuit can be.
Retired Professional Railwayman (1981-2023); Pway & S&T (1981-88); Former Signalman/Signaller/ Signalling Trainer (1989-2023) [AB, TCB, Mechanical, NX, WestCad, Hitachi SARS]; Railway Historian (esp.SER, LCDR); Member of The Permanent Way Institution..
Last edited: 16/05/2017 at 13:48 by Giantray
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