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Metrolink head-on crash in Chatsworth
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[QUOTE]Originally posted by Geoff M: [QB] [QUOTE]Originally posted by rresor: [qb] To Geoff M: With due respect, I think US signal systems are a bit more sophisticated than those in Britain, and it would take more than one loose wire to produce a false clear. Also, the signal at CP Topanga is an "absolute" (home) signal whose normal (default) display would be red. If it were green continuously, someone certainly would have noticed.[/qb][/QUOTE]I think we'll have to agree to disagree on which is more sophisticated! As I said before, we require double cutting which means it takes TWO loose wires doing the same false feed in order to get a wrong side failure. Double cutting in this instance means both the positive and return wires both being put through relay contacts, compared to your traditional method of just switching either one or the other, usually the positive. All I was trying to do was explain how, if the onlookers were correct, a green could have been shown whilst the data loggers recorded a red being commanded. If the engineer did call an advance approach then that sounds like the signal *should* have been red. I don't have the typical US signalling circuits with me at the moment but I agree that with your statements about false indications being "literally impossible". FWIW, US signalling circuits are very similar to UK circuits in the foundation levels, even using the same relay contact names. Where they sometimes differ is that of aspect control. That one possibility is as I stated above - far beyond the reach of data loggers as it's the final link between interlocking and signal lamps. An unlikely scenario but one which has happened before. And I'm not theorising that this is what happened here; just trying to explain one possible reason IF the onlookers were correct in their statements that the signal was green. Geoff M. [/QB][/QUOTE]
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