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Post by GentlemanJim on Apr 9, 2013 19:33:58 GMT
I vaguely remember these and think they only 'popped up' to allow trains to get closer albeit at reduced speed. Maybe someone can offer a better explanation.
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Post by Nortube on Apr 9, 2013 23:13:27 GMT
It's possible that their operation / purpose may vary depending on the location.
Morden had a pop-up signal - A728 - added in order to allow the starters to clear a few seconds earlier if there was a train ahead. Effectively, this was just an additional signal added to increase line capacity (of sorts) by breaking one section into two sections.
A728 is situated just before the NB tunnel mouth. In order to avoid the risk of a SPADded station starter due to read through (the starter is red but the A728 is green as long as its section is clear), the signal was made a pop-up.
When the starter is on, A728 has no aspect. When the first pair of wheels goes over the blockjoint of a station starter, A728 will show a green aspect, or a red aspect if there is a train still in its section. The aspect on A728 will go out when the last pair of wheels passes over the blockjoint at A728.
There have been times when the aspect has remained on. Whilst technically this is a defect, the aspect itself would show red or green as appropriate and as far as a driver was concerned, there was nothing wrong with the signal.
If following right behind a train, especially if the train in front was moving slowly, there was always a chance that A728 could still be showing red when the train reached it and the driver would have to stop. This could cause gapping problems if a train departing platform 2 was held at it as the whole train could become gapped, especially if one or more shoes were missing. Indeed I've seen this happen on at least two occasions. Both times this meant a long delay while depot staff brought the jumper leads from the depot because the platform jumper leads had been removed.
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Post by railtechnician on Apr 10, 2013 6:40:46 GMT
I vaguely remember these and think they only 'popped up' to allow trains to get closer albeit at reduced speed. Maybe someone can offer a better explanation. I dunno, you guys and your slang! I thought I'd need a translator but I'm beginning to learn another dialect of motorman speak! What you're calling a 'pop up' signal is known 'in the trade' as a suppressed signal ! The reason for suppression is to avoid giving a particular view of the state of the road ahead until a suitable time. For instance there is such a signal in the Heathrow loop which has a suppressed red aspect because it can be seen from so far away, that part of the loop being dead straight for a long way, and a motorman approaching it at distance might slow down unnecessarily upon seeing it too early. The loop has a comparatively high line speed (I can attest to that personally having tried unsucessfully to unlock the clamplock units at 10W in the days before the existence of T5 with trains bearing down upon the turnout relentlessly during a failure) and an unsuppressed signal would lead to unwarranted reduction as a result of false perception. Suppressed signals are suppressed red aspects, the green aspect is not suppressed. The green aspect will always be displayed when it is selected but the red aspect will only be displayed after an approaching train has dropped onto the approach track to complete its selection. What you have described is not a defect! Suppressed signals can catch out newbie linemen when doing their routine night maintenance, if they don't know the area and haven't read the prints it can be a trial working out what is 'wrong'! Even experienced linemen can be caught out in unusual circumstances! |
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Post by 21146 on May 10, 2013 12:55:44 GMT
There were 'pop up' signals on the District Line west of Aldgate East. I never quick saw the purpose of them as my Road Trainer told me "If you can't see them it means they'll be red".
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Post by Deleted on May 10, 2013 15:52:18 GMT
There were 'pop up' signals on the District Line west of Aldgate East. I never quick saw the purpose of them as my Road Trainer told me "If you can't see them it means they'll be red". A bit like the one in the Heathrow loop mentioned upthread. The green is not suppressed, only the red, so if you don't see the green you know it'll pop up red. Rare to see it on now though with the 10 minute headway since T5 opened. |
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