If a train loses the air in its brakes, it brakes sooner

A 1,600-tonne freight train is coming down a mountain pass and its brake hose bursts. Intuition says that train has just lost its brakes and is gone. The opposite happens: it brakes. And it brakes harder and faster than in a normal brake application.

That is the design, and it was chosen on purpose a century and a half ago. Here is how it works, in five steps:

1. The locomotive carries a compressor that pushes pressurised air into a pipe running the whole length of the train, wagon by wagon, joined between cars by hoses. On freight trains that pipe works at around five bar: five times the pressure of the air around you.

2. Every wagon has its own air reservoir, a small tank filled from that pipe. That is where the braking force of that wagon lives, stored and waiting. It does not come from the locomotive: it is already under the car.

3. Between the pipe, the reservoir and the cylinder that pushes the shoes against the wheel sits a part called the triple valve. Its job is to compare two pressures: that of the pipe and that of the reservoir.

4. While the pipe is full, the valve keeps the brake released. Air, here, is not what brakes: it is what stops the brake from biting. It is the reverse of what a car pedal does.

5. When the pressure in the pipe drops, the valve lets the wagon's reservoir discharge into the cylinder and the brake applies. If the driver lowers the pressure a little, it brakes a little.

And here is why a rupture brakes sooner than a driver does. When the driver wants to brake, he vents air from the head of the train and the drop travels backwards along the pipe at about 250 metres per second; on top of that the fall is gradual, because he is metering it. When a hose bursts halfway down the train, the pipe empties at that point to atmospheric pressure and the drop runs both ways at once, from the middle. Each valve gets the full collapse, which to it is the emergency application, and dumps its wagon's reservoir into its own cylinder at full force, without waiting for air to arrive from the locomotive. That also settles the third fault of the 1869 brake: the tail no longer lags behind the head, because every car brakes with the air it was already carrying.

The interesting thing is that it was not invented this way. George Westinghouse patented his first air brake on 13 April 1869, aged 22, and it was precisely the brake anyone would design: patent 88,929 describes a pump driven by steam from the boiler, a reservoir under the locomotive holding air at some 80 pounds per square inch — about 5.5 bar — and a pipe carrying that air to the cylinders of the cars to push the shoes. Air arrives, brake grips. It was called the straight air brake and it was an enormous leap: until then, braking a train meant several brakemen running along the roofs turning wheels by hand, car by car, while the driver whistled.

It had three faults. First, if the pipe broke, nothing braked. Second, if a train split on a gradient, the loose wagons were left with no air and nothing to hold them. Third, slower to notice: on a long train the air took time to reach the rear cylinders, so the tail kept pushing while the head was already braking.

Westinghouse did not improve that brake. He inverted it. Patent 124,404, of 5 March 1872, adds an auxiliary reservoir on each wagon, filled from the main one and usable on its own; with the triple valve, developed in those same years, the braking signal stops being air that arrives and becomes air that leaves. The result has its own name in engineering: fail-safe design. The fault fires the mechanism instead of disarming it.

It is worth knowing the exact limit, because there is one. This brake protects against a rupture and against an uncoupling, which happen in seconds. It does not protect against a slow leak: a train standing with the compressor off loses air for hours, the reservoirs equalise with the pipe and the brakes end up releasing themselves. That is why the rulebooks do not allow a train to be parked trusting the air brake: you have to wind on the mechanical hand brakes of a number of wagons, with the same handle the brakemen used in 1869.

Patent 124,404 turned 154 last March, and the mechanism it describes — a reservoir and a valve under every wagon, and a pipe that brakes when it empties — is still fitted to almost every freight train in the world.