Train brakes by lack of pressure?

[Enthalpy]

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 are pressed by a spring and need pressure to open.

 

Could this be any different in Canada or the US?

Thanks!

[Enthalpy]

This site tells that automatic air brakes are standard in Canada, and act when pressure drops:

http://www.railfame.ca/sec_ind/technology/en_2002_AutomaticAirBrake.asp

 

Wagons have also valves to isolate them from the brake line, and to purge their pressure reserve so they can be moved if isolated, for instance to sort wagons using a small hill.

Edited July 11, 2013 by Enthalpy

[Greg H.]

The issue is that, there are actually two air brake systems on most modern North American trains.

 

You have the individual reservoir on the car which is what actually provides the clamping power for the brakes. By default, this tank holds the brake closed by supplying constant pressure from what's called the train line. A separate line, called the brake line, uses compressed air supplied by the locomotive to engage and disengage these tanks, which is what releases or applies the brakes.

 

The problem is that the locomotive is also responsible for keeping the air brake reservoirs on each car fully charged (via the train line). If a car needs to sit for a long period of time without a powered locomotive, manual hand brakes still need to be set (they're the large wheels you see on the ends or sides of the railway cars). Without a running locomotive, eventually the charged air system will leak away enough pressure that it can't hold the brakes closed. If the manual brakes have not been engaged at that point, the whole thing is basically at the mercy of gravity and friction.

 

If you're interested, here's a recent article from popular mechanics that explains it in more detail without being too technical.

http://www.popularmechanics.com/technology/engineering/understanding-a-runaway-train-how-do-air-brakes-work-15678938

Edited July 22, 2013 by Greg H.

[Enthalpy]

Leaking air tanks would eventually leave the train without brakes... But:

- After an hour? I can't imagine it from sound hardware.

- On nearly ALL wagons? A few wagons without leak would have stopped the train.

- Normal practice, at least in Europe, demands to put a wedge on the track (I haven't found the English word) before abandoning a train.

 

Let's hope the investigation will find convincing answers.