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| Re: Real Long Read On NYCPropulsion/Braking (333639) | |||
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Re: Real Long Read On NYCPropulsion/Braking |
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Posted by Stephen Bauman on Sat Nov 4 09:03:54 2006, in response to Re: Real Long Read On NYCPropulsion/Braking, posted by randyo on Sat Nov 4 03:17:55 2006. What I noticed was that the signals on the bridge before replacement were only about 250 ft apart which may have been adequate for the BU elevated cars which originally operated over the bridge in 5 car train lengths but were totally inadequate for later equipment which consisted of 8 car steels and later 8 car R types.The signals are actually between 264 and 271 feet apart. The block before the collision was 270.35 feet. The distance between the tripper and the back of the stopped train was approximately 288 feet. (NTSB Report) The question is how far the train will travel before coming to a stop. Its length or weight is immaterial. The Standards had an emergency braking rate of 3.0 mph/sec; the BU cars had an emergency braking rate of 2.0 mph/sec. The R1/9's duplicated the Standards' performance. Later cars are supposed to have an emergency braking rate of 3.2 mph/sec. The simple relation between initial speed and stopping distance is: d = 0.735 x V2/a, where d is the stopping distance in feet, V is the initial velocity in mph a is the braking rate in mph/sec. One can also turn this relationship around to determine the maximum velocity for a given stopping distance and braking rate. That relation is: Vmax = sqrt(1.36 x a x d). Applying this relation for a 264 foot block we get the following maximum safe velocities for the following equipment: BU - (2.0 mph/sec): 26.8 mph Standards, R1/9 - (3.0 mph/sec): 32.8 mph SMEE - (3.2 mph/sec): 33.9 mph. The NTSB conducted tests over the Williamsburg Bridge after the collision. Their test train attained a speed of 33 mph, when it reached the tripper. The test train required 364.5 feet to stop for a braking rate of 2.2 mph/sec. The NTSB also ran a full service braking test. The train stopped in 162.2 feet from the tripper, using full service braking for a braking rate of 4.93 mph/sec. Neither the BMT nor its successors the B of T and the TA, however, seemed to feel it necessary to alter the signal spacing on the bridge to provide the necessary stopping distance behind the longer trains that were now being operated. The length of the train is irrelevant. The track circuits will detect the presence of the leader's last car, whether that train is 250 or 536 feet long. The reason for the close spacing is twofold. First, maximum speed on the bridge is restricted because of the grades. This means that spacing between trains will be reduced on that section from a straight flat track that led to it. Second, the Williamsburg Bridge was a major trunk line, carrying 27 tph in 1949 and probably more before the 14th St Line's completion. These two criteria determine the block length, not the train length. As a result, a collision such as the one described would have been inevitable and I'm surprised one did not take place sooner. I think I've demonstrated that it could not happen sooner because the BRT, BMT and BOT operated operated equipment that adhered to its emergency braking rate spec. The problem on the Willy B was not necessarily a problem of inadequate braking capability of the rolling stock, although that entered into it, but rather the retention of 1900s signal technology on a 1990s system. Sorry, I don't think you can rationalize this failure with the "if we only had CBTC" excuse. :=) |
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