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| Re: (interlocking) TA is obsessed with CBTC, and ''New'' tech for no reason. (580156) | |||
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Re: (interlocking) TA is obsessed with CBTC, and ''New'' tech for no reason. |
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Posted by Jeff H. on Mon Mar 3 19:36:13 2008, in response to Re: TA is obsessed with CBTC, and ''New'' tech for no reason., posted by Stephen Bauman on Mon Mar 3 08:46:02 2008. See around curves? I'd do it with mirrors (or an occasional repeater).Mirros? That's a new one! Anyway, the advantages of a cab signal display were argued and won 70 years ago. Besides curves (both horizontal and vertical), there is glare, snow/rain/fog, etc. Not to mention lamps being out (and before you even say it, LED lights blow out too). Of course there is a standard for Lamp Out detection and downgrading the previous signal, but it is not used in transit applications because of the relatively low speeds and short stopping distances. You mean a tower operator can throw a switch open, while a train is barreling towards it? Just what was the design spec for that interlocking? Uh oh. You might want to read a primer on interlockings, because you seem to have a common misconception. Simmons Boardman has a few books available which you can order online. Some are pretty technical with actual circuits, others are more from a railfan perspective. Let me explain this in terms of electro-mechanical interlocking because that is most intuitive. Say we're on a GRS pistol grip machine (model 2 or 5). In order to move a switch lever, all of the signal levers which protect that switch must be pushed in, ALL the way. If any such signal lever is out, it mechanically prevents the switch lever from being moved in either direction. When a signal lever is pulled all the way out, it allows the signal to clear in the field if all other conditions are met (track occupancy, stop arm has driven down, all switches in the route are over, locked and in correspondence with their associated levers) NOTHING locks the signal lever from being pushed in. The tower operator can drop a signal at any time, including when the train is 3 feet in front of it. However, it would clearly be a bad design if, in such a case, one could immediately unlock the protected switch lever, because the train would be unable to stop in time and you would throw the switch under the train. Therefore, the signal lever has a locking detent when you push it in. It stops just short of the fully-in position. If there is no train approaching the signal (based on track circuit occupancy) then the lever goes right in and you can change the lineup. But if there is a train approaching, there is a timer which starts to run when the lever has been pushed partly in. The time value depends on the length of the approach track circuit and the assumed maximum speed of the train, plus a margin. When that timer finishes, the lever is unlocked and can be pushed fully in, thus allowing the route to be changed. The switch lever is also electrically locked from moving when the short track circuit which covers the switch area is occupied. This is known as "detector locking". There is a manual bypass procedure in case the track circuit has failed. |
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