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I can see that possibility of a near zero safety factor causing a push-pull test to be unreliable, and maybe that is part of the reasoning by the TSB in saying that test is unreliable.  It seems to me that the issue of a safety factor partly depends on which of these two versions of the test is used: 1)      The train is actually pushed or pulled to see how the handbrakes hold. 2)      The air brakes are simply released on a standing train to see if the handbrakes hold.   People here have described both procedures.  With #1, you can at least judge how much safety factor there is by the way the push or pull feels. With #2, there is no way to know how much safety factor there is.  With #2, if you set 15 handbrakes, release the air, and it holds, that only tells you that 15 is enough.  If 10 would have held it, then you have 5 extra as a safety factor.  But there is no way of knowing how many extra brakes have been set.  It might be that 14 would not have held it, so in that case, you would only have one extra brake as a safety factor.  It could be an even closer margin than one whole brake.  What if the train would have rolled up to the point of having 14.9 brakes set? Is it safe to leave a train parked on a grade just hanging by a thread so to speak?  The equipment cools off, the ambient air temperature changes, the solar heating changes, and wind can kick up.  If a train is on a grade with the smallest amount of handbrake safety margin, a cooling contraction of the rails could send a small impulse through the cars standing on those rails.  The slightest slack effect created by the impulse of contracting rails could break the holding friction of one or more handbrakes.  Once that breaks, the friction begins to fall off.  And once that happens the necessary friction will never be regained.  The train will roll, and the friction will continue to fall off as the speed increases.  But I am not sure if the Transportation Safety Board of Canada is referring to an uncertain safety factor when they say that it is impossible to know that the train is secured by using the push-pull test.  What I find curious about their statement is that they link it with the condition of being on a “high grade.”  I don’t understand why that would matter.  If you push and pull on a grade, the resistance is going to be higher pushing or pulling up-grade that it will be down-grade.  But why not just do the test for the downhill direction since that is the direction that the handbrakes need to work against?  I tried to ask the TSB what they meant by saying that it is impossible to know whether handbrakes would hold from conducting a push-pull test.  They would not return the call, and I have since read that they will not engage in any discussions with the public about railroad train brake systems.  That is not just in regard to discussion about the MM&A runaway, but the whole train brake subject in general.  However, I would guess that the restriction of discussing brakes is based on the fallout of the MM&A runaway.
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