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[quote user="Paul_D_North_Jr"] See Sections 1.14.2 - Net Braking Ratios, 1.7 - Brake Shoes and Friction Fade, 2.7 - Braking Capacity, and 4.3.1 - Safety Concern - Performance Standards for Composition Brake Shoes, of the TSB report on the 29 June 2006 CN (ex-BCR) runaway and derailment, at:  <a href="http://www.tsb.gc.ca/eng/rapports-reports/rail/2006/r06v0136/r06v0136.asp#1">http://www.tsb.gc.ca/eng/rapports-reports/rail/2006/r06v0136/r06v0136.asp#1</a>  These sections detail the various differences in the amount of retarding force required from each brake shoe, the causes of each, and how those standards has evolved.  In particular, Section 1.14.2 - Net Braking Ratios states that each car is required to provide a braking force of at least 8.5% = 0.085 (1 / 12th) of the car's weight.  For a 72-car train on a 1.2% grade, the gravity force downgrade is the equivalent of 0.864 cars (ignoring static resistance to rolling, and the weight and braking effect of the locomotives).  At 0.085 of a car's weight for each car's brakes, to develop 0.87 cars' worth of resistance would require a little over 10 - say, 11 cars - to have their handbrakes applied. Most importantly, note that there is NO Factor or Margin of Safety in these calculations or this resulting number.  Even just 1 of those 11 cars without brakes fully and effectively applied would be enough to allow that train to start rolling.  Practically, I would suggest a factor of safety of at least 1.5 to 2.0, which would require from 16 to 22 cars to be 'tied down'.    That goes to the result of the "test": What is a successful test ?  When all of the wheels slide as the locomotive tugs downhill ?  How long/ far after the locomotive stops pulling must the train have come to a halt ?  A short time/ distance would correspond to a high factor of safety; a longer time/ distance - or no set standard - would be associated with a marginal number of brakes set, right on the edge of being just barely enough, or just a couple brakes too little.  Even a train that has a barely adequate number of brakes applied may be deemed to have "enough", even though it is right on the edge of being uncontrollable.      - Paul North.  <div style="clear:both;"></div> [/quote] Paul, You make some interesting points about the interpretation of the push-pull test.  Those variables must be part of the reason why the TSB has decided that the push-pull test is unreliable.  If they are correct, what does this mean in light of the fact that this has been standard routine in North American practice for so long?  If the procedure is flawed, has it just been a matter of luck and good judgment that it works so often?
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