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The air brake protects against a runaway train

like the one that barreled through the small town of Lac-Megantic, Quebec, on Saturday and derailed, its cargo of crude oil from North Dakota bursting into flames and engulfing the community's downtown in an inferno that killed at least 15 people. In the immediate aftermath many are asking how the train's air brakes could have failed so disastrously. But how do they work, anyway?

In some ways, a train's air brake is a lot like the brake on your car. In your car, slamming on the brake pedal transfers your foot pressure–through brake fluid–to a piston, which clamps a pair of pads tightly over the brake disk in your car's wheel. Air brakes also use pressure to squeeze the moving railcar's wheels to a standstill. But instead of brake fluid, they use compressed air.

Rex Beatty, a conductor for 35 years and the president of the Teamsters Canada Rail Conference, says that today each train car has an isolated air tank filled with pressurized air. As a failsafe, this air reservoir is constantly supplying the pressure needed to brake the wheels. "So when you pull the brake on the train, you're just using that reservoir in the car," he says. "That's how you get the brakes."

Essentially, the brakes are always on. For the train to actually move, the operator has to disconnect this air tank. He or she does this by pumping air into a separate line called the brake line, which switches a valve to separate the tank from the brakes. And as soon as the brake line stops delivering pressure, the valve automatically flips back and reconnects the air tank, engaging the brakes.

The beauty of this system is that, in normal circumstances, it requires pressurized air for the train to move. In the old days, a single leak could make a train lose all its pressurized air, and it wouldn't be able to stop. Today "if you lose all your air," Beatty says, "the brakes will automatically [activate]." And if one of the air tanks somehow malfunctions, the rest of the train can compensate because each car has its own.

So how could the disaster in Canada have happened?

The only danger with air brakes is if all the air tanks lose pressure. Beatty says that because of unavoidable leakage, the tanks are always gradually seeping air. "Air can leak around the piston and can leak through the various connections between each car," Beatty says, "but there will eventually be leaks." Normally, the train's engine is constantly resupplying the railcar's tanks with air, and this slight leakage isn't a problem.

But trains parked for an extended period of time rely on hand brakes which, just like the old days, are manually engaged at the back of each railcar. This anchors the locomotive without relying on its airbrakes, which–without a running engine–are gradually losing their power. And when the train starts back, the operator won't move the train without making sure the air tanks are refilled.

Investigators have not released details about the Quebec train derailment, but it's probable that for some reason, the air brakes were left on too long without engaging the individual cars' hand brakes. When firefighters responded to a blaze and shut off the train's running engine, the locomotive might have eventually lost pressure in the air tanks—and lost its stopping power. This could have caused the train to roll away and gain speed, resulting in the disaster this weekend.

The true cause of the Quebec train derailment won't be known until officials complete their investigation, but properly used, air brakes have been reliably stopping trains for almost 150 years. "I found the air braking system on trains very safe and effective," Beatty says, "but like everything else, it involves certain procedures that need to be followed."

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