How freight trains brake on mountain grades

HostIf you were standing by the tracks in a mountain pass, you might expect to smell that sharp, burnt scent of metal on metal as a massive freight train comes down the grade. But most of the time, the hardest working parts of the system aren't even touching the wheels. How does something that heavy keep from just turning into a runaway heater?

GuestIt really is a constant battle against the mountain. Think about a train that weighs maybe fifteen thousand tons. When that much weight starts rolling down a steep hill, gravity is basically trying to turn all that mass into pure speed as fast as it can. Now, if the person driving—the engineer—tried to slow down using just the regular brakes, like the pads on your car, they would be in trouble in a matter of minutes. Those metal parts would hit over a thousand degrees. At that heat, the metal surfaces can actually get soft and liquidize a little bit. It creates this slick, lubricant effect where the brakes just slide instead of gripping. They call it brake fade, and once it starts, those friction brakes lose all their stopping power. The train basically becomes a runaway.

HostThat sounds like a nightmare. So if they can't rely on those pads rubbing the wheels, what's actually holding back all that weight?

GuestMost of it's done with magnets and electricity. It's called dynamic braking. See, most of these big engines are diesel-electric. They have a big diesel engine that turns a generator, which then sends power to electric motors at the wheels to make the train move forward. But when the train goes downhill, the engineer flips the process. They switch those motors into generator mode. Now, the momentum of the heavy wheels is forced to spin the motors, which creates a huge amount of electrical resistance. It's like an invisible drag that slows the wheels down without any physical parts rubbing together.

HostWait, so the wheels are essentially charging the engine? But if you're generating all that electricity, where does it go? You can't just store a lightning bolt's worth of power in a battery every time you go down a hill.

GuestYou don't store it. You burn it off. On the roof of the engine, there's a massive bank of wire grids that acts like a giant toaster. All that electricity created by the wheels is sent to those grids, where it turns into pure heat. They have these huge cooling fans that blow that heat away into the air. So the train is using its own weight to make power, then turning that power into heat and blowing it off the top of the engine. It keeps the heat completely away from the wheels and the actual brake shoes.

HostThat's a lot of work for the engine to do. But these trains are huge, sometimes a mile long. Does the engine do all that braking by itself, or do the cars in the back help out?

GuestThe cars definitely have to help, but they use a different system. It's an air brake system, but it works the opposite way you might think. In a car, when you step on the pedal, you're pushing fluid to the brakes to make them grip. On a train, they use a fail-safe system. They keep the entire air line that runs through the train under high pressure just to keep the brakes in the off position.

HostHmm, so the air pressure is actually holding the brakes back? That seems like it would be hard to manage.

GuestIt's all about safety. Every single car has what they call a triple valve. This valve is the brain of the car's brakes. It senses the air pressure in the line. When the engineer wants to slow down, they actually reduce the air pressure. The triple valve feels that drop and lets air from a small local tank on each car push the brake shoes against the wheels. This is why it's a fail-safe. If the train ever breaks apart and the air hose snaps, the pressure in the line drops to zero instantly. When that happens, every car on the train slams its brakes on at the same time. They call it the big hole. It ensures that a broken train doesn't just roll away down the mountain.

HostOkay, but if you have the engine braking with magnets at the front and then a hundred cars using air brakes in the back, how do you keep the whole thing from just buckling? It's not one solid piece of metal.

GuestThat's the hardest part of the job. A train is really just a long series of weights connected by metal couplers that have several inches of wiggle room, or slack. If the engineer uses the engine brakes too hard, the heavy cars at the back will slam into the front of the train. That can cause the cars to jackknife and pop right off the tracks. But if the back is braking harder than the front, the train stretches out, and you could actually snap a coupler and break the train in two. The engineer has to carefully balance the magnetic drag at the front with just the right amount of air pressure in the back to keep the slack bunched up or stretched out perfectly.

GuestIt's a delicate balance where the engineer is managing fifteen thousand tons of steel using nothing but magnets and air to make sure that mile-long chain of cars stays in one piece all the way to the bottom.

HostThe tracks stay quiet and the air stays clear because those giant fans on the roof are doing the work that friction never could.