Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Train brakes by lack of pressure?

  1. Jul 11, 2013 #1

    An attempted explanation for the tragedy of Lac-Megantic is that the only running locomotive was shut down by the fire brigade when extinguishing it, and as this locomotive stopped to provide pressurized air to the train, the wagon's brakes opened and the train ran away.

    Which I can't understand, because at least here in Europe, for over a century and exactly for the cited explanation, all brakes need pressure to open, not close.

    It's not a straight vacuum brake: each wagon has a reserve of air pressurized by the locomotive when the train runs, and this reserve lets the wagon brake it the pressure drops in the control line, for instance if wagons detach from the locomotive. Of course air can leak, but this would be required from most wagons at the same time for the train to move.

    Could this be any different in Canada or the US?
  2. jcsd
  3. Jul 11, 2013 #2
    The brakes on railroad cars in the U.S. and Canada indeed are applied by air pressure and released by the lack of pressure. There are reservoirs of air and the air does slowly leak out of the reservoirs, releasing the brakes. There are manual brakes that can be applied if needed.

    Although this seems counter intuitive, there is a reason for it. When a train goes to a yard, the cars are uncoupled and sorted out to different trains depending on their destination. They do this by pushing the cars, one at a time up a small hill or hump and allowing the car to roll down the other side while a switchman switches it onto the proper track. How would they do this if the brake applied automatically?

    It seems that the problem was that the train engineer was not present and thus couldn't apply the manual brakes when the engine was shut down.
    Last edited: Jul 11, 2013
  4. Jul 11, 2013 #3
    Thank you!

    The patent claimed and exploited by Mr Westinghouse in the US describes a brake that releases by pressure and brakes by pressure drop. He sold enough of them to grow a big company. When was that abandoned?

    Sorting out wagons by a hill is done in Europe as well, so there is some sort of trick.

    Anyway, seeing the result, it's time to improve that - provided brake by pressure is used.
  5. Jul 11, 2013 #4
  6. Jul 11, 2013 #5
    I check this point precisely because I won't take as secure information what the train company tells and the Press repeats.

    Here's a doc telling that automatic air brake is the standard in Canada, and that brakes apply by decreasing pressure in the brake pipe:
    All other docs tell the same, for North America as for Europe - except Spain and Portugal which seem to have vacum brakes.

    As for the sorting hill: wagons have several manual controls, some to isolate the brake pipe, others to purge the pressure reserve in order to release the brakes despite the brake piepe is disconnected:
    http://florent.brisou.pagesperso-orange.fr/Freinage wagon.htm

    "Des poignées installées de chaque côté du véhicule sont reliées par des câbles à la tirette de purge du distributeur, de manière à pouvoir desserrer les freins après isolement."
    translates to:
    "On each side of the vehicle, handles are connected by cables to the purge valve, so as to release the brakes after isolation."
  7. Jul 11, 2013 #6
    Pressurize the system, lock it off with a valve.
  8. Jul 11, 2013 #7


    User Avatar

    Staff: Mentor

    What fraction of valves do you think are actually 100% closed when the handle is turned to the closed position? And what fraction of non-permanent fittings have absolutely no leaks?
  9. Jul 11, 2013 #8


    User Avatar
    Gold Member

    I recall when having to move boxcars years ago that the brakes on some of the boxcars had been left on when uncoupled, and the resevoirs had to be purged to release the brakes before movement. Left standing on their own the brakes would release by themselves as the air leaked out.

    This is a report of an accident in 2008 in BC that resulted in car derailment in British Columbia in what seems to be a case similar to what happened in Lac Megantic minus the horrific damage.
    A trifle more than halfway down is an explanation on air brakes.

    Service application of brakes is achieved by a gradual reduction in brake pipe air pressure until the point called equalization at which the brakes are fully applied. At equalization and full service of the brakes, the auxiliary air resevoir and brake cylinder pressures are equal. Reducing brake pipe air pressure achieves no more braking force.
    For this train, brake air pipe pressure was to be maintained at 80 psi. Full service braking would be at 64 psi.

    A rapid reduction in brake pipe air pressure results in emrgency braking. Below 45 psi brake pipe air pressure the brakes may not fully engage.
  10. Jul 12, 2013 #9
    It appears you are arguing both sides. In this quote you say that brakes apply by decreasing pressure.
    But in the next quote you say that the pressure must be purged in order to release the brakes.

    And again
    I work with the railroads and as an RF engineer I don't have first hand knowledge of this issue, however I have contacts at all the major RRs and did speak to a number of engineers about this very issue in order to understand it better.
  11. Jul 12, 2013 #10
    The pressure in the reserve brakes when there is no pressure in the brake pipe.

    This is intended, so that uncoupling the wagons from the locomotive lets them brake automatically. To move the uncoupled wagons, one has to manually purge the reserve.
  12. Jul 12, 2013 #11


    User Avatar
    Gold Member

    Did you actually not read what I wrote or just simply bypassed the explanation?
  13. Jul 12, 2013 #12
    257Bits was right on. I talked to my father about this last night. He has been a locomotive engineer for about 20 years and he described a similar system. He also maintained that the fire department should have had the locomotive engineer there when they shut off the locomotive. He also mentioned that where he works there are 2 locomotive operators: a conductor and an engineer. The conductor is responsible for applying hand breaks to the locomotive and the engineer is responsible for making sure the train is safe before they leave. A simple safety measure. Obviously we dont know the specifics of this situation though.
  14. Jul 13, 2013 #13
    wow I hadn't heard the news until now... tragic incidence... I still can't believe how much damage it's caused and I feel terribly sorry for the people...

    However, it seems that a hell lot of things went wrong... as is usually the case BUT could all these be purely coincidential? No air-brakes, no hand-breaks, fire and the train carrying oil? uhm... it seems to me that they should investigate the possibility of a sabotage...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook