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Derailments Caused By Emergency Braking?
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<p><span style="font-family:verdana,geneva;">Paul,</span></p> <p><span style="font-family:verdana,geneva;">I had not pondered slack action during emergency braking until getting into this thread. I had always visualized it fundamentally running in with brake service applications unless the train is pulled to keep it stretched during braking. But when I consider emergency application from the head end, I am not so sure what happens.</span></p> <p><span style="font-family:verdana,geneva;">I see a lot of reference sources listing the speed of emergency propagation as well as the speed of brake setup on each car. Some references talk about how these numbers have changed over time and differ with different brake pipe pressures.</span></p> <p><span style="font-family:verdana,geneva;">Hilton says that slack runs in or out at 200-400 feet per second. I don’t think that statement is adequately qualified to stand just like that. He goes on to say, “If it takes 90 seconds for brakes to go on in the whole train, the train will have contracted 100 feet or more in the interim, mainly in the first few seconds.”</span></p> <p><span style="font-family:verdana,geneva;">Again, I believe there are missing components in that analysis. Hilton is referring to 100 cars, with one foot of slack in each. If the train is 5000 feet long and the slack runs in at 300 feet per second; that will take 16.6 seconds.</span></p> <p><span style="font-family:verdana,geneva;">When he says 90 seconds for the brakes to go on in the whole train, he means completely set up. But the emergency propagation rate is 900 feet per second, so that means that when the engineer puts the brakes into emergency, it will only take 5.5 seconds until the last car goes into emergency. But it will take another 84.5 seconds until all of the brakes in the train fully apply maximum braking pressure, and the last car to fully apply will be the last car in the train.</span></p> <p><span style="font-family:verdana,geneva;">So that means that each car requires 84.5 seconds to fully build up maximum braking effort once the application begins.</span></p> <p><span style="font-family:verdana,geneva;">Hilton says that the slack will completely run in “mainly in the first few seconds.” I don’t see that. If it requires 84.5 seconds to fully apply, how much braking are you going to develop in just “the first few seconds”?</span></p> <p><span style="font-family:verdana,geneva;">But the larger question is this: If the brakes on the hind end start to apply only 5.5 seconds later than the brakes on the head end—and if it takes 84.5 seconds to fully apply, once a brake begins to apply—what is the slack really inclined to do?</span></p> <p><span style="font-family:verdana,geneva;">Is that 5.5-second difference in the initiation of such a long application phase really enough to cause the degree of run-in shock that Hilton describes? As the run-in begins, all the cars will have initiated some degree of braking. And as the run-in wave moves back, the braking on the stretched cars will be further increasing.</span></p> <p><span style="font-family:verdana,geneva;">Hilton says, “The cars will have banged into one another with an impact which at its worst is equal to a switching impact of 14 mph.” From that figure, he calculates a force of 937,860 foot-pounds.</span></p> <p><span style="font-family:verdana,geneva;">For this scenario to make sense, enough braking will have had to occur on the bunched part of the head end to decelerate by at least 14 mph while hind end cars are coming on with no deceleration yet. That is the part I can’t see. By the time the first cars have dropped 14 mph, the rear cars will have also started to decelerate. So I am not convinced that there will ever be a 14 mph discrepancy anywhere in the train. </span></p> <p><span style="font-family:verdana,geneva;"> </span></p>
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