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| Re: TA is obsessed with CBTC, and ''New'' tech for no reason. (578700) | |||
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Re: TA is obsessed with CBTC, and ''New'' tech for no reason. |
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Posted by Jeff H. on Sat Mar 1 00:08:11 2008, in response to Re: TA is obsessed with CBTC, and ''New'' tech for no reason., posted by trainsarefun on Fri Feb 29 22:43:03 2008. Suppose further that with CBTC, trains are allowed toapproach the wall at 8th Av up to (and in fact at) the speed limit that would enable an emergency stop short of the wall. That would be called suicide. There is a margin of safety built in to the movement authority limit, of course. Any system which relies on fixed trips is what is called an "intermittent train control" system. The design margins must be higher because the designer only has the ability to control the train by tripping it at certain points. This makes things like speed control very awkward. Traditional cab signal systems with ASC are continuous train control, as is CBTC. The primary theoretical advantage of a communications based approach is that the train has the ability to talk back to the wayside system and indicate its ability to stop. This must be integrated with the propulsion and braking controls in a vital (life-safety critical) manner, which makes it a difficult and expensive design. I believe this "stop-assure" is of dubious value since the NYCTA went with their traditional friction brakes only during emergency approach, and thus there isn't too much variability in stopping distance due to car equipment factors (roadbed conditions are another story). In the NYCT implementation of CBTC, interlockings are done almost like traditional relay-based interlockings, with fixed home signals and trips. There was never any claim of improved interlocking performance, other than the automatics approaching the plant. As Baumann has repeatedly explained and calculated, terminal capacity in a 2-track stub is all about how quickly you can get a train to clear the crossover and normal up the switch. Incidentally, part of the long-term signal strategy is to get rid of pneumatic switches. If switch operating time is an important concern, those air switches have a few second advantage over their electric counterparts. Traditionally, timers were designed to "prove" that the train is under control and operating below a certain speed. It was assumed that, e.g. when approaching a bumping block, the motorman would not decide to commit suicide and wrap up the ccontroller the instant that the last stop arm goes down. The new signal design guidelines pretty much make that assumption, to the point that they are attempting to apply continuous train control principles to an intermittent system. That makes the cost of doing any new signal work so high that CBTC starts to look good. |
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