Prototype Railroading: How Air Brakes Work

A few days ago, we talked about how the air brakes on trains came to be.  To many people, they are complicated systems with air flowing back and forth, which by some miracle brings thousands of tons of train to a controlled stop.  Hopefully by the end of the day, they'll be understood a little better.  First of all, let's go over the basics again.

The air brakes on a train operate on the principle that unequal pressures will equalize.  That is to say, if a higher and a lower pressure meet, the lower pressure will increase and the higher pressure decrease until the two are equal.  The brakes in most trains worldwide operate by forcing compressed air through a pipe the length of the train.  The pressure inside that pipe puts air into the system, as well as engages the system when the pressure is changed.  As pressure drops, or when it is lost completely, the brakes apply.  When pressure is added to the system, the brakes release.  They say that a picture is worth a thousand words, so enjoy them.  They're not to any scale, they are simply for reference.

On each locomotive, there is an air compressor, which compresses air for the brakes, as well as other air powered functions on the locomotives, such as the sander, horn, and the wipers.  The air leaves the compressor and goes to the main reservoir, which consists of two large air tanks on the locomotive, used to store air.  From the main reservoir, where the pressure is typically between 120 and 140 pounds per square inch (psi), the air passes through a regulating valve and into the brake system.  The regulating valve can be adjusted to higher and lower settings, but it controls what pressure the brakes will operate at.  For most freight trains, it is 90 psi.  Passenger trains vary, depending on who operates them, but they are usually between 90 and 105 psi.  Think of the main reservoir as a lake, with the regulating valve acting as a dam.  When more water is needed out or the lake, the dam can be opened more, but when the water level downstream of the lake is high enough, the dam can be closed, to allow less water to pass through.

Once air leaves the main reservoir and passes through the regulating valve, it enters the brake system.  The air pipe that goes the length of the train is called, not surprisingly, the brake pipe.  As air enters the system at the locomotive, it flows back, until it has filled the brake pipe in every car, ad the diagram above illustrates.  Now, we are going to take a look at an individual car for a minute.  Looking at the small picture will help understand the big picture.

On each car, there are a few components of the brake system.  The brake pipe runs the length of the car, with a hose on each end for coupling it to the next car.  At each end of the car, there is a valve, called an angle cock, which allows the hose to be closed off if no cars are coupled.  This prevents the pressure from simply escaping out the end of the train.  Connected to the brake pipe on each car are two air reservoirs.  One is for normal braking, and the second one is for emergency braking.  There is a control valve, which actually translates the change in air pressure into a brake application or release, and there is a brake cylinder, which actually provides the braking force.  Braking force is transferred to the wheels by the means of a brake shoe, a composite pad that the brake cylinder pushes against the wheel tread.  (Note: On passenger trains, typically a disc and caliper system is used instead of a shoe against the wheel system.)  Let's look at a single car for a minute.  The following diagram is very basically what the brake system looks like.

The control valve is what allows everything to happen.  In a minute, we will talk about how that works.  The control valve operates solely on air pressure.  There are no electronics that make the control valve work.  It is simple changes in pressure that make it do everything.  When the brakes are released, it allows air to flow from the brake pipe, into the air reservoirs, keeping them full of air, or charge up.  When pressure drops in the brake pipe, the control valve transfers air from the reservoirs to the brake cylinder, thereby applying a braking force to the wheels on the car.  The pressure transferred to the brake cylinder is equal to the pressure change in the brake pipe.  So, if the engineer makes a ten pound reduction, that is to say if the engineer lets 10 psi out of the brake pipe, the control valve will transfer 10 psi to the brake cylinder.  If the engineer then makes another five pound reduction, 5 psi will be added to the air already in the brake cylinder, generating more braking force, and stopping the train sooner.  When the pressure in the brake pipe goes up, the control valve lets the air out of the brake cylinder into the atmosphere, and it then lets the air reservoirs refill from the brake pipe.

Inside the control valve are actually a series of piston driven valves.  The pistons move based on differences in pressures, and when they move, the change the position of valves, which control air flow within the brake system on the car.  As complicated as it sounds, it is actually surprisingly simple, and quite ingenious.  One piston controls air movements under normal braking.  A second piston is to allow an emergency brake application to happen very quickly.  Below are two diagrams of the inside of a control valve.  First we will look at the control valve in the released position.

When the control valve is in the released position, the pressure on either sides of the piston is equal.  The pressure on the left side of the piston is always the same as the brake pipe, and the pressure on the right side is the same as the air reservoir.  When the brakes are released, the increasing pressure of the brake pipe pushes the piston into the above position.  This slides the valve assembly into position to release all pressure from the brake cylinder, releasing the brakes.  It also pushes the piston against a choke valve.  Once that choke valve opens, air pressure can build up inside the air reservoirs, so that there will be pressure ready when the brakes are needed again.  When the engineer lowers the air pressure to apply the brakes, the piston moves, as shown below, which in turn moves the valve assembly.

When the brakes are applied, the pressure on the left side of the piston is lower than the pressure on the right, forcing the piston to the left.  When the piston moves to the left, the valve assembly moves and directs air from the reservoir into the brake cylinder.  The brake cylinder transfers that pressure to the brake shoe, and generates braking force.  Because the pressures on either side of the piston are attempting to equalize, the pressure on the right side, and in the air reservoir, will never drop below the pressure on the left side.  It is simply not possible.  If the engineer reduces the pressure in the brake pipe, on the left side of the cylinder, by ten pounds, then the pressure in the reservoir, connected to the right side of the piston, will also drop by ten pounds.  Those ten pounds of air can only possibly go to the brake cylinder, and so ten pounds of air are used to generate braking force.  Should the engineer reduce the pressure further, then the pressures will again equalize, sending more air to the brake cylinder, and applying the brakes harder.  When the engineer needs to release the brakes, he moves the brake valve in the locomotive to release, and air from the main reservoir flows into the brake pipe.  When that happens, the pressure on the left side of the piston is higher than on the right, forcing the piston to the right, and releasing the brakes.

The diagrams above only cover normal service brake applications.  Above is the basic system developed by George Westinghouse, in the 19th century.  Because the system works on balancing pressures, having 90 pounds of air available does not actually mean you have 90 pounds of braking force.  The pressures will only equalize, and therefore the pressure that will go to the brake cylinder is considerably less than actual pressure in the reservoir or in the brake pipe.  It would be impossible for the reservoir to completely empty under these braking conditions, unless the engineer emptied the brake pipe.  That is how an emergency application is made.  However, as this system developed, it was realized that the engineer would only usually empty the brake pipe when an emergency arose, and he needed to stop as quickly as possible.  So, an additional system was developed, which is designed to work only when the brake pipe pressure is lost quickly.  Inside the control valve, there is another piston to control that system.  The emergency brake system, just as the normal service brake, is charged when the air is released.  When the choke valve is open, as shown in the first diagram, air can flow into the two air reservoirs.  That builds up a pressure in both the normal service reservoir and in the emergency reservoir, so the air is there when needed.

In the diagram above, we can see that the emergency brake system operates in a similar manner as the service brake.  Again, a piston moves back and forth and controls what the emergency brake does.  However you will notice a few key differences.  There is no choke valve on this piston, because the emergency reservoir is filled only when all brakes are released.  There is no exhaust port on this control valve, because otherwise pressure would be able to escape from the brake cylinder anytime the emergency brakes had not been applied.  This is the same brake cylinder as the service brake uses.  All air exhausted from the brake cylinder goes through the other valve.  Also, this piston has a hole in it.  The hole actually allows air to flow slowly from one side of the piston to the other.  This is what prevents the emergency brake from applying every time pressure drops on the left side, which is whenever the engineer makes a normal brake application.  In normal brake applications, air pressure changes slowly enough that the air can flow through the hole.  When the engineer needs the emergency brake, he empties the brake pipe, and all the air pressure is lost suddenly.  When that happens, the change is too rapid for the air to flow through the hole, so the piston moves, triggering the emergency brake application, shown in the diagram below.

Just as in the service brake, the piston moves to the left, moving the valve into position to allow air to flow from the emergency reservoir to the brake cylinder.  When the emergency brake is applied, the service brake is also fully applied, so the air from both cylinders is directed into the the brake cylinder, which allows for maximum braking force.

Everything described above takes place on each and every single car in a train.  The emergency valve and the service valve are both located in the same unit, and together they are known as the control valve, because they control the flow of air on the individual rail cars.  The flow through the brake pipe is established by the engineer, who has a valve in the locomotive.  He can exhaust air into the atmosphere, resulting in a drop in pressure on the brake pipe, and a brake application.  He can place the brake valve in release, allowing compressed air to flow into the brake pipe, increasing the pressure, and releasing the brakes.  The way the system works allows the pressure to be used to generate braking force, but also allows for any sudden loss in pressure to bring the train to a safe stop, rather than lose the brakes.  The basics of the system have remained relatively unchanged since its invention in the 19th century, however other features have been added to the system.  Features exist now which to help speed up the release of the brakes from car to car, so that the rear end of the train is not still braking while the front is released, as air flows back to the rear.  An emergency application is also expedited by more advanced features, so that an entire train, over a mile long, can have the emergency brakes apply almost simultaneously from end to end.  The entire system is purely pneumatic, and does not rely on any electronics.  Electronics have been added, primarily to passenger cars, to achieve non slip braking, basically anti-lock brakes for a train, but the brake systems even on those cars will function as intended if the electronics fail or are disconnected.  Despite being an old system, it is an extremely reliable system, and there has simply been no need or reason to develop a new brake system.

Note:  All diagrams in this post were created by James Ogden and are copyrighted.  They may be used for informational, educational, personal, or other nonprofit purposes, with proper citation.  Any commercial reproduction or publication outside of these terms is forbidden without written permission.