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| Re: TA is obsessed with CBTC, and ''New'' tech for no reason. (579687) | |||
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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 Sun Mar 2 21:23:30 2008, in response to Re: TA is obsessed with CBTC, and ''New'' tech for no reason., posted by Jeff H. on Sun Mar 2 01:18:48 2008. I think what you are missing is that SB is comparing actual CBTC performance to conventional signal performance in a different time period.Several things have happened since the peak service levels of the 1950s which have cut into the ability of the fixed wayside system. Most of them have to do with trusting, or rather NOT trusting the person in the cab. Before you reach that conclusion, might I suggest you perform the following thought experiment. Decide on a minimum separation between trains, where the follower will not encounter any red or yellow signals and the follower does not have to come partially into a station. Next, remember with the scenario of the follower approaching a station occupied by its leader that the minimum distance will be achieved when the departing leader reaches the speed of its approaching follower. First: determine how long and how far the leader will travel in reaching this velocity. Second: assume the follower is the requisite distance behind the leader at the instant the leader has reached the same velocity as the follower. Third: calculate the time it takes the follower to come to a complete stop in the station. Fourth: add the times from the first and third steps, add 30 seconds for dwell time within the station to come up with a figure for the minimum headway. If this minimum headway is comparable to those from the halcyon days of the 1950's, then keying-by and partial station entry are not required for such headways. Let us begin by making some realistic assumptions for the minimum distance and the maximum velocity. I'm going to assume 1200 feet and 30 mph. The 1200 foot separation should insure that the follower never encounters a yellow signal (let alone a red signal) approaching the station. It also means that when the follower enters a 600 foot long station the leader is another 600 feet beyond it. Feel free to suggest another value, remembering that the object is to guarantee only green signals in coming to a stop. The 30 mph seems reasonable in today's environment. Next, we need to calculate the time and distance taken by the leader in accelerating to 30 mph. I can think of 3 methods to accomplish this. The first is a pre-computer engineering approach of assuming a uniform acceleration remembering that all we are interested in is where and when. The second is the multi-million consultant approach of taking the aceleration profile of the equipment and numerically integrating it with a Runga-Kutta program to get the leader's speed and distance traveled as a function of time. The third is to take a stopwatch and time trains leaving stations. I've tried all three approaches. For ease let's take the first approach by assuming an average acceleration of 2.0 mph/sec. Thus the leader will reach 30 mph in 15 seconds and travel 330.75 feet. Let's assume 600 foot trains. We want the follower to be 1200 feet behind the leader's rear car. So the follower is 869.75 feet from the station entrance and 1469.25 feet from its stopping point at the front of the station. Now let's bring the follower to a complete stop. We could assume a well-trained T/O who would apply the full service braking rate of 3 mph/sec and take 10 seconds and 220.5 feet to stop the train. However, let's assume a timid T/O who brakes at an average rate of 2.0 mph/sec. The train will 15 seconds and 330.75 feet to stop. This means that the train will continue traveling at 30 mph for 1138.5 feet and take 25.8 seconds. Thus the total time for the follower to come from 1200 feet behind its leader to a complete stop is 40.8 seconds. This brings the minimum headway with 30 seconds of dwell time to: 15 + 40.8 + 30 = 85.8 seconds. This is equivalent to 41.95 tph. Clearly, keying by and station time were not necessary for the 30, 32, 34 and 36 tph peak service levels of the 1950's (through early 1970's). They occurred but they were an anomaly not a necessity. That still leaves door enablers and pointing. Strictly speaking, these activities do not add to dwell time but to braking time. (The definition of dwell time is the interval from when the doors open until when the train starts leaving.) Allow a generous 5 seconds total for the T/O and C/R to agree on which side to platform and for the C/R to give the monitor board the finger. That brings the minimum headway up to 90.8 seconds or 39.6 tph. That's close enough to 40 tph for government work. |
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