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| Signal visibility and change of aspect 29/01/2012 at 08:18 #28379 | |
maxand
1637 posts |
I thought I understood how approach locking works, but I still have trouble getting my head around ACOA (adverse change of aspect). After playing around with various sims trying to emulate this, it seems there are two situations in which COA (change of aspect) plays a part. Let's consider 4 main signals along a track (with no platforms involved): A, B, C, D. A route is set A-B-C-D, so in three-colour aspect signalling the aspects are green-green-yellow-red. A train enters A-B, then C is replaced, so B changes to yellow in front of the train. Approach locking then imposes a 120 second delay on releasing C-D (not really sure why, since the train has all of B-C in which to stop, and the driver knows the next signal will be red) but there is no ACOA because the train is still in A-B. On the other hand, if the train were in B-C when C was replaced, this would not only invoke approach locking but also an ACOA as the next signal is the one replaced, which is fair enough. What I also fail to understand is why trains in SimSig never overshoot a replaced signal, no matter how fast they are going or how close to the signal they happen to be. Is it because this is just a simulator? Also, how early should a driver be able to see the next signal along his route? If the blocks between signals are long I assume he may not be able to see the next signal immediately after passing one, so with no cab radio, how would he know it had been replaced to danger? Must there be a "chain of visibility"? I thought that was old-fashioned now in view of higher speeds. It would make more sense to me if approach locking started its timer only when the train enters the last TC before the signal, rather than what appears to happen, namely as soon as the train enters the block. Here's one example from Royston.  K986 is replaced as soon as 1C77 enters the block behind it. 1C77 is only within the first TC of that block, which could be long enough for its driver not to be able to see K986 from his present position, yet replacing the signal triggers an immediate ACOA (you can't see it, but K986 is flashing). On the other hand, if emergency replacement occurs just as the train comes up to the signal, it always manages to stop without overshooting, thus the overlap is never exercised. How does this compare with real life? Thanks in advance. Last edited: 29/01/2012 at 08:22 by maxand Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 08:27 #28380 | |
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Stephen Fulcher
2088 posts 
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The signal that the train saw before would have been green in your picture, therefore the driver would not be expecting to see a red, only a yellow or green. This is working correctly.
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| Re: Signal visibility and change of aspect 29/01/2012 at 09:04 #28382 | |
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urbanfox
8 posts |
Rather than start a new topic, I guess I can add this here. Was playing Sheffield last night and had cleared 218, but then pulled it off in favour of letting a train cross to go down to Rotherham. I got a message saying the train approaching 218 had recieved an ACOA, however the train was approaching 432 - which remained green throughout. Is this as a result of the next signal (428, which did revert from green to double yellow) being within the line of sight? Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 09:13 #28383 | |
Peter Bennet
5419 posts |
Within SimSig the default sighting distance is 300m so if a train is heading towards a G which changes to Y it'll only report an adverse change of aspect if the train is 300m or less from the signal- or whatever distance has been set by developer. However, as Stephen says, if the train is expecting a specific aspect because of the previous aspect was less than G and the aspect actually found is more restrictive than expected then that is also an adverse change of aspect. Arguably the wording of the message is wrong in that the train may "find" an adversely changed aspect rather than received - as in witnessed- the change but that's probably not high on the fiddle list. Peter I identify as half man half biscuit - crumbs! Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 09:29 #28384 | |
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Peter Bennet
5419 posts |
On the matter of over-running signals, the Sim platform will not allow it- try doing an E button replacement immediately in front of a train and the train will stop dead. I seem to recall some time back it was possible, specifically where a train entered the Sim close to a signal that was R- it skidded past it. Peter I identify as half man half biscuit - crumbs! Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 11:11 #28388 | |
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Firefly
521 posts |
Quote:(not really sure why, since the train has all of B-C in which to stop, and the driver knows the next signal will be red)Many (most these days) signals in real life time out regardless of whether a train is approaching or not. It's called "Approach Locked When Cleared" . By omitting "lookback" data you save lots of testing time, lots of data writing and lots of processing power. We only tend to put "Comprehensive Approach Control" (the type that does lookback) on important junctions where the signaller is likely to want to change their minds and the timeout delay would cause a loss in operational flexibility. Quote: It would make more sense to me if approach locking started its timer only when the train enters the last TC before the signal, rather than what appears to happen, namely as soon as the train enters the block.You definitely couldn't do that. At 90mph the signalling braking distance required is 1537m assuming a level gradient. This a distance where every type of train can sight the first caution signal, react to it, apply the brakes and bring the train to a halt using full service brake (not emergency brake) allowing some margin for error (I'm not sure what margin they use).  So in my drawing above if a train had not yet passed 3 signal you could safely return 1 to danger without a need to time out. (if comprehensive approach locking installed)  Once the train had passed 3 signal we still have the opportunity to revert No.1 to danger and although the driver will probably see No.2 change from Green to Yellow it's not unsafe because you still have 1537m stopping distance, however a driver will stop the train, report the irregularity (fill in forms and cause much delay). The point that No.1 signal MUST become approach locked is the point when the driver is unlikely to re-look at No.2 signal to notice that it has reverted to yellow. In reality that is about 200m on the approach to No2 however we rarely have track circuit breaks at that point so for ease we simply lookback as far as No 3 signal and approach lock the signal if any train is between No 1 and No 3 signals. In your example of just looking back one track circuit section the train will be doing 90mph and may not sight No 1 signal until it's 300m from it (if it's on a bend or there's lots of bridges and overhead wires in the way). Therefore by the time the driver reacts to the red aspect and makes an emergency brake application it will still SPAD by around 1000m if not more. This is why the route must have remained locked to ensure that you couldn't move points, lift barriers or route another train across the path of your train. As far as SimSig is concerned Geoff took the decision many years ago that SimSig trains will never SPAD no matter what the signaller does, therefore SimSig trains will stop on a sixpence if required. Post has attachments. Log in to view them. Last edited: 29/01/2012 at 11:15 by Firefly Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 11:29 #28389 | |
lazzer
636 posts |
" said:Also, how early should a driver be able to see the next signal along his route? If the blocks between signals are long I assume he may not be able to see the next signal immediately after passing one, so with no cab radio, how would he know it had been replaced to danger? Must there be a "chain of visibility"? I thought that was old-fashioned now in view of higher speeds. As a driver I can assure you that whatever anyone else is told (or whatever the official rules are) we are not made aware of any such thing as "how early we should see our next signal". Instead, it's more a case that the signals are spaced far enough apart so that we can get the train from linespeed to zero before we reach the the red. I'm just running though the line between Reading and Didcot in my head, and of the ten signals on the Down Main between Tilehurst station and SB901 (see SwinDid sim), none of them are visible at the same time as each other. Despite the sim showing a lovely straight line, the route curves alternately left and right as it snakes through the Thames Valley. Further, the signals are all three-aspect up to the signal before SB901 (the one that flashes double yellow for a route across Didcot East Junction), so at the linespeed of 125mph you would need to get the brakes in pretty quick on sighting a single yellow around a curve. (In the up direction, the signal just after Pangbourne station can show single yellow on occasion, and that makes your heart rate go up, I can assure you ...) To get back to your original point, if the signaller puts one of those ten signals to danger ahead of me, then it's brown trousers time when I come round the corner at 125mph and see it at red! I would assume that the stopping distance beyond the signal is sufficient for me to stop before I hit something! On a relative theme, the only SPAD I've ever had was a Cat B at the bottom of Dauntsey Bank (east of Chippenham) due to a track circuit failure, and I saw that one go red on me from three-quarters of a mile away. That didn't stop me having the sweaty palms and increased heart rate as I went past it at 85mph! Log in to reply The following user said thank you: Prof Jolly |
| Re: Signal visibility and change of aspect 29/01/2012 at 11:34 #28390 | |
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TomOF
452 posts |
Interestingly on some stretches of the East Coast main line around Peterborough (Tallington springs to mind) you can see many signals ahead owing to the straightness of the line. I also don't know how true this is but I was told that Signal P440 (up fast at Peterborough station ) only had a sighting time of 7 seconds at 105 mph owing to various low bridges and OHLE structures. Last edited: 29/01/2012 at 11:43 by TomOF Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 11:46 #28391 | |
Steamer
3997 posts |
To summarise: 1. If a route is cancelled that causes a signal that a train is approaching to change to a more restrictive aspect, approach locking (generally 120 seconds) will apply. 2. If a route is cancelled, causing signals to change to a more restrictive aspect, however no train is approaching those signals, approach locking will not apply (there are exceptions on the Sheffield, CScot and Trent Simulations) 3. Drivers will report seeing an ACOA if they find a signal at a more restrictive aspect than they were expecting (for example, passed a signal at G, and found the next one at R) 4. However, if the signal the driver is approaching changes from G to Y on a 3-aspect line, or G to YY on a four aspect line, he will only report an ACOA if he sees the signal change (this is either the default 300m, or location specific). maxand said: What I also fail to understand is why trains in SimSig never overshoot a replaced signal, no matter how fast they are going or how close to the signal they happen to be. Is it because this is just a simulator? This was a deliberate decision on the part of the developers, explained in this Wiki page. "Don't stress/ relax/ let life roll off your backs./ Except for death and paying taxes/ everything in life.../ is only for now." (Avenue Q) Last edited: 29/01/2012 at 11:50 by Steamer Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 13:45 #28394 | |
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maxand
1637 posts |
Thank you all for your kindness in providing such detailed replies. I had no idea this was such a deep rabbit hole. Thanks Steamer for your concise summary. On reading around, a good definition of the difference between approach locking and comprehensive approach locking (CAL) came from Chris Bellett, here: Quote: Approach locking is normally initiated once the signal controls are set ready to clear the signal, but before the signal actually clears (the signal clearing then proves the approach locking has been applied). In the event of the route being cancelled then a timer operates which holds the route locking (time delay dependent on various conditions).Another one from IRSE exams, with a typically respectful flavour to it: Quote: So "Approach Locking" is the maintenance of the locking after a signal has been replaced to danger until such time as it is safe to release it. Having given a promise to the driver by clearing a signal, we must honour it until the driver has been given the opportunity to respond to a change in aspect.And the official definition in RSSB's GKRT0060.pdf - not as clear but still intelligible: Quote: Approach locking: The application of locking to a route that shortens the movement authority previously given to a train. The locking is released after a time interval duringAfter all this it seems to me that SimSig uses comprehensive approach locking, since one can cancel a route from two signals away if no train is approaching. If Approach locked when cleared was used, every time I cancel a route there would be a timeout, train or no train. Am I correct? Last edited: 29/01/2012 at 13:46 by maxand Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 14:08 #28395 | |
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Late Turn
699 posts |
" said:After all this it seems to me that SimSig uses comprehensive approach locking, since one can cancel a route from two signals away if no train is approaching. If Approach locked when cleared was used, every time I cancel a route there would be a timeout, train or no train. Am I correct? Almost - a couple of sims (for example, the area around Clay Cross on the Trent sim, as Steamer suggests) have signals without comprehensive approach locking (to match reality). Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 14:08 #28396 | |
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Peter Bennet
5419 posts |
" said:SimSig supports both, comprehensive being the default. Cowlairs on CSCOT has no comprehensive locking. Peter I identify as half man half biscuit - crumbs! Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 14:08 #28397 | |
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sloppyjag
480 posts |
Not an expert, so may be wrong here, but approach locking (whatever flavour) would only apply to the signal held at red (until approach timer completes) so cancelling the route from this signal before it changes to yellow/green would not be changing it to a more restrictive aspect so no time-out would apply. (I think?)
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| Re: Signal visibility and change of aspect 29/01/2012 at 14:20 #28398 | |
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Steamer
3997 posts |
" said:Not an expert, so may be wrong here, but approach locking (whatever flavour) would only apply to the signal held at red (until approach timer completes) so cancelling the route from this signal before it changes to yellow/green would not be changing it to a more restrictive aspect so no time-out would apply. (I think?)You're right- approach locking will not apply if the route is cancelled before the signal clears. Out of interest, why has signal 450 (at Alderwake Jn.) on the Sheffield sim been given a 240 second none-comprehensive time-out? "Don't stress/ relax/ let life roll off your backs./ Except for death and paying taxes/ everything in life.../ is only for now." (Avenue Q) Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 14:38 #28399 | |
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Firefly
521 posts |
Quote:After all this it seems to me that SimSig uses comprehensive approach locking, since one can cancel a route from two signals away if no train is approaching. If Approach locked when cleared was used, every time I cancel a route there would be a timeout, train or no train. Am I correct?Spot on :woohoo: Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 14:41 #28400 | |
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Firefly
521 posts |
Quote:why has signal 450 (at Alderwake Jn.) on the Sheffield sim been given a 240 second none-comprehensive time-out?That's probably what it was in reality. FF Log in to reply |
| Re: Signal visibility and change of aspect 29/01/2012 at 22:29 #28417 | |
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maxand
1637 posts |
Thanks again for all your helpful comments. 
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| Re: Signal visibility and change of aspect 30/01/2012 at 06:56 #28424 | |
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maxand
1637 posts |
One more question: I've discovered in Royston that cancelling any routes set from a shunt signal in a loop or siding to a main line signal automatically starts the approach locking timer, whereas this does not apply in reverse (setting a route from main line signal to shunt signal), nor to shunt signals on a main line. This happens even when there are no trains in the area, and no goods trains about to enter from a siding.  For example, cancelling K244-K976 invokes the timer, whereas cancelling K246-K976 occurs immediately. I can't recall seeing this mentioned in any of the documentation. This is obviously deliberate, but I can't understand why. Thanks in advance for your help. Last edited: 30/01/2012 at 06:57 by maxand Log in to reply |
| Re: Signal visibility and change of aspect 30/01/2012 at 06:59 #28425 | |
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Peter Bennet
5419 posts |
It's because the siding has no Track Circuit so the signal system it does not know whether there is a train there or not- so assumes there is. Peter I identify as half man half biscuit - crumbs! Log in to reply The following user said thank you: maxand |
| Re: Signal visibility and change of aspect 30/01/2012 at 07:30 #28426 | |
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maxand
1637 posts |
Ah, thanks for that.
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| Re: Signal visibility and change of aspect 30/01/2012 at 08:13 #28427 | |
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clive
2799 posts |
" said:Interestingly on some stretches of the East Coast main line around Peterborough (Tallington springs to mind) you can see many signals ahead owing to the straightness of the line.What got me interested in signalling was, as a boy, standing on the Up platform at Leigh-on-Sea and seeing about 6 signals as I looked towards London. We often caught the second train to London because the first would be going via Tilbury Riverside and the second one would arrive before it. So I could see the train departing in the distance with the four-aspect sequences changing behind it. What annoys me in retrospect is that I was often at Watford Junction yet I never paid attention to the New Line signals! Log in to reply |
| Re: Signal visibility and change of aspect 30/01/2012 at 08:28 #28428 | |
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clive
2799 posts |
Okay, a quick wash-up on the topic. (1) It is a matter of SimSig policy that trains never pass a red signal without authority. If this means breaking the laws of physics, so what? We don't care. (2) Firefly has explained approach locking in great detail. SimSig offers both comprehensive and "when cleared" approach locking (and it's also possible to create custom locking). (3) The train simulation module is what generates the "received an adverse change of aspect at signal" messages. These are generated when the next signal in advance of the train is lit and its aspect changes to a lower value and * EITHER the train is within the sighting distance of the signal (default 300m, but can be changed on a per-signal basis) * OR the new aspect is not one that can be predicted from that of the last signal that the train passed. If the latter applies but not the former, this is technically "will receive an adverse change" but I can't be bothered to change the message (though I've added it to the bug list). The messages are suppressed if the train is unpowered, it is in the process of "jumping" to a chained simulation, there is another train "hiding" the signal from us (used to deal with permissive sections), or for about 15 seconds after the previous such message. A train will only get one such message per signal that isn't in sight, irrespective of what changes happen, but will get more than one if the signal is visible for the second change. Last edited: 30/01/2012 at 08:31 by clive Log in to reply |
| Re: Signal visibility and change of aspect 30/01/2012 at 08:32 #28429 | |
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Jsun
212 posts |
Additionally, subsidiary routes at main signals are approach released from red. So when the route is cancelled there is no need to hold the interlocking to time as the train is expected to stop anyway.
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