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| Re: TA is obsessed with CBTC, and ''New'' tech for no reason. (580600) | |||
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Re: TA is obsessed with CBTC, and ''New'' tech for no reason. |
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Posted by Stephen Bauman on Tue Mar 4 15:39:17 2008, in response to Re: TA is obsessed with CBTC, and ''New'' tech for no reason., posted by Jeff H. on Mon Mar 3 23:00:05 2008. But you can't have your cake and eat it too. If you want schedule speed and you want trains to reach balancing speed or nearly so between stations,Trains seldom reach balancing speed; they seldom exceed 35 mph even on express runs. They used to have half mile markers and one of my pastimes was to time express routes. The Flushing Line between 61st and Junction drag strip made it to 37.5 mph on a good day with R12/14/15's. The sole exception: a friend of mine did time the Rockaway line at 52 mph. The minimum block length should be greater than the emergency stopping distance. These distances are: Emergency braking stopping distance with 35% safety factor: @25 mph - 193.1 ft; @30 mph - 279.1; @35 mph - 379.8; @40 mph - 496.1; @45 mph - 627.9. I concluded that a 400 foot block length was not unreasonable and that would insure that a follower 1200 feet back would see only green lights. However, if you remain unconvinced, I'll eat the extra 13.3 seconds that an 1800 foot spacing would add to the minimum headway. But if they are behind schedule, it is very difficult to make up time. That depends on two items: amount of time to be made up and the sources of variation. The reason for feedback at each station is to keep deviations from schedule to under 15 seconds. Variations in dwell time, caused by passengers holding doors, is cited as the principal component. There are other sources that are well within the TA's ability to control. Mr. Selkirk did not exaggerate, when he stated that he could make up time by braking and accelerating hard. I've measured stopping and starting times for successive trains at the same station. These measurements were taken when all the signals were green. These measurements showed that difference between min and max stop/start times was at least 10 seconds. Moreover, timidity on the brakes was a fairly good predictor of timidity on the throttle. Thus, the total variation on stopping and starting at stations was around 20 seconds. The second place to make up time is reducing dwell time. Dwell time is composed of three components: doors open/passengers crossing the door threshold; doors open/no people crossing the threshold and doors closed/train stationary. Clearly there is some margin for reducing the last two items without injuring passengers. I have not measured what I speculate to be the greatest source of operator variability: travel time between stations. In short, I think there are areas where time can be made up - provided it is not an excessive amount. |
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