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View Diary: More Mystery Surrounds Canadian Ghost Train: Where are the locomotives? (160 comments)

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  •  Your title, which is (0+ / 0-)
    When pressure is OFF, brake is ON
    isn't fully compatible with the (more correct) statement in the body of your post:
    the brake is OFF until pressure is applied,
    I think what confuses people is that the locomotive supplies "line pressure" which does two things

    1) it pumps up a reservoir in each car, which can be used to apply the brakes on that car when needed

    2) it acts as a sensor that in effect keeps point number 1 from happening while a train is operating normally.

    But when line pressure is lost (say, the locomotive is turned off or the cars become detached from the locomotive),  the line pressure is lost, triggering the reservoir in each car to release air to the brakes on that car and thereby apply the brakes.

    Therefore in a way it is true application of the brakes requires loss of pressure but that only applies in a indirect manner.  More directly air pressure (from the reservoir) IS REQUIRED to apply the brakes.

    OK, once the brakes have been applied this way, the reservoir is sufficient (usually!) to stop the train.  However, the air will slowly dissipate (in minutes or hours) - therefore if the handbrakes have not been applied and the train is on a grade, it will eventually start moving again.

    That's what happened here!

    Everybody in these threads keeps talking about how the brakes function in stopping the freakin' train.  That really has only minimal relevance (because the engineer had already done that!).  What IS relevant is how the train started moving again . . ..  ((which is explained a paragraph or two above).

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