Hi! 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? Thanks!