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| London Transport (uk.transport.london) Discussion of all forms of transport in London. |
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#19
May 31st 05, 09:29 PM
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Why can't LU cope with a signal failure?
"Tom Anderson" wrote in message h.li... On Tue, 31 May 2005, Brimstone wrote: Boltar wrote: They tried the system where trains set off at 2 minute gaps without the hindereance of signals about 100 years ago.....too many crashes....You might be a bit late for a meeting or dinner...but this is safe. I'm not suggesting ignoring signals, I'm just trying to figure out the logic behind the system they use now. Signal has failed so go slow across the next 2 working signals. Don't get it. Signals don't fail to green (supposedly) so if the signal after the failed one is green why on earth go slow past it, why not just go normal line speed? It just seems OTT. Because of something called an "overlap". In order to provide protection for the next train ahead the signal is positioned before the start of the track section it controls access to. The length of the overlap varies according to the weight and speed of the trains plus an allowance for bad weather. If driver's resumed normal speed after passing only one signal at clear or caustion there might be another train or other problem withing the overlap. Then no one gets home for tea. According to http://www.trainweb.org/tubeprune/signalling1.htm, signals are linked to the overlap between them and their section, as well as to the section itself (the overlap, being part of the previous section, is also linked to that section, of course). If that's true, there can never be anything between a green signal and the end of its section, in the overlap or not. Thus, it is always safe to go up to full speed at a green signal. No? tom PS This is eerily reminiscent of concurrent computer programs. Things queueing up on a critical section, regulated by wait and signal operations. Such programs are notoriously and fiendishly difficult to write correctly - or to read! PPS Got as far as drawing this much of a highly entertaining diagram before deciding text alone would do: /----O /----O /----O ==== ==================== ==================== ======= ^....^ Thought i'd include it in case anyone feels like drawing pictures and wants somewhere to start! tom -- power to the people and the beats My brain hurts now...... Mal |
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