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If I understand how the brakes work, they are engaged by air pressure from a reservoir on each car which is divided into two parts for service braking and emergency braking.  Before the train gets underway, the brake system is pressurized and the reservoirs fill with air.  When air pressure along the train is slowly released below a certain pressure service braking is engaged.  When all the pressure in the air line is rapidly released the emergency reservoir will apply additional force to the brakes.  Over time air leaks from the brake system on each car.  As air pressure in the reservoirs drops the force of the brakes drops. 

On perfectly level ground that might not be a big deal but on a slope gravity opposes the braking force.  The steeper the slope and the heavier the car the greater the force due to gravity. As air leaks from the system eventually the braking force reduced to less than what's needed to oppose the force of gravity and the train will begin to roll.  As the train picks up speed braking forces are not just opposing the static force of gravity but the kinetic force which takes even more braking force to overcome.

Now, if I understand the system correctly the only way to re-engage the air brakes on the cars to stop the train is to re-pressurize the air line to refill the reservoirs to full pressure releasing the brakes then re-engage them by releasing the pressure -- slowly for service brake or rapidly for emergency brakes.  There are two things conspiring it takes a long time to recharge the brake system and during that time the train is accelerating so there is an increasing amount of kinetic energy for the brakes to stop. 

In an extreme scenario the speed increases so the kinetic energy is beyond what the air brake system can stop -- this happened in the Cajon pass derailment featured on an episode of Mayday.  What likely happened in the case of the CP derailment, and now I get why the TSB was particular about not calling it a runaway, is the train simply ended up going too fast for sharp curves accelerating while the engineer was desperately trying to get the brake system up to pressure to control the speed of the train.  Until the brake system was fully pressurized all that would have been available to slow or stop the train would be the independent and dynamic brakes on the 3 locomotives which would be woefully insufficient.

While the distributed locomotives may have helped get pressure to the brake system faster it wasn't fast enough for a train accelerating on a steep slope heading toward a sharp curve just minutes away.

The lesson is don't park a heavy train on a steep slope for a long time relying on air brakes.  With some statistics predicting the rates at which rail car air brakes lose pressure engineers (the white collar kind) could come up with an equation or model to predict what the maximum safe time a train could stop on a given slow with air brakes.  Hopefully this incident will trigger the TSB and Transport Canada to do the research and come up with that so dispatchers and engineers can make better informed decisions in similar circumstances.

 

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