A moving train contains energy, known as kinetic energy, which needs to be removed from the train in order to cause it to stop.  The simplest way of doing this is to convert the energy into heat.  The conversion is usually done by applying a contact material to the rotating wheels or to discs attached to the axles.  The material creates friction and converts the kinetic energy into heat.  The wheels slow down and eventually the train stops.  The material used for braking is normally in the form of a block or pad.

The vast majority of the world's trains are equipped with braking systems which use compressed air as the force to push blocks on to wheels or pads on to discs.  These systems are known as "air brakes" or "pneumatic brakes".  The compressed air is transmitted along the train through a "brake pipe” or, in North America, a “train line".  Changing the level of air pressure in the pipe causes a change in the state of the brake on each vehicle.  It can apply the brake, release it or hold it "on" after a partial application.  The system is in widespread use throughout the world.  

The Air Brake

The air brake is the standard, fail-safe, train brake used by railways all over the world.  In spite of what you might think, there is no mystery to it.  It is based on the simple physical properties of compressed air.  So here follows a simplified description of the air brake system. 

The diagram in Figure 1 shows the principal parts of the air brake system.

Figure 1: Schematic of air brake system. The air is drawn into a compressor and stored in a main reservoir at 7-10 bar (100-140 lbs/ Compressed air from the main reservoir is distributed along the train through the main reservoir pipe. On each vehicle, the pipe is connected through a triple valve to an auxiliary reservoir which stores air for use on that vehicle's brake system. The flow of air between the auxiliary reservoir and the brake cylinders is controlled through the triple valve or distributor. The control of the distributor is achieved by varying the pressure in a second pipe called the brake pipe, which is connected to a brake valve in the drivers cab. Increasing the pressure in the brake pipe causes the brakes to release, while decreasing the pressure causes the brakes to apply. Diagram: Author.

Driver's Brake Valve

The driver controls the brake by means of a valve in the cab.  The brake valve will have (at least) the following positions:  "Release", "Running", "Lap" and "Application" and "Emergency".  There may also be a "Shut Down" position, which locks the valve out of use. 

Figure 2: A cutaway Westinghouse No. 4 driver’s brake valve with the equalising valve on the right and the feed valve below. This is the brake valve that was my introduction to train braking. Photo:

The "Release" position connects the main reservoir to the brake pipe.  This raises the air pressure in the brake pipe as quickly as possible to get a rapid release after the driver gets the signal to start the train.

In the "Running" position, the feed valve is selected.  This allows a slow feed to be maintained into the brake pipe to counteract any small leaks or losses in the brake pipe, connections and hoses.

"Lap" is used to shut off the connection between the main reservoir and the brake pipe and to close off the connection to atmosphere after a brake application has been made.  It can only be used to provide a partial application. A partial release is not possible with the common forms of air brake, particularly those used on US freight trains.

More Information

Vacuum Brake description from the Railcar website.

The Westinghouse Air Brake a 19th century description from the Catskill Archive

Train Equipment

"Application" closes off the connection from the main reservoir and opens the brake pipe to atmosphere.  The brake pipe pressure is reduced as air escapes.  The driver (and any observer in the know) can often hear the air escaping. 

Most driver's brake valves were fitted with an "Emergency" position.  Its operation is the same as the "Application" position, except that the opening to atmosphere is larger to give a quicker application.

To ensure that brake pipe pressure remains at the required level, a feed valve is connected between the main reservoir and the brake pipe when the "Running" position is selected.  This valve is set to a specific operating pressure.  Different railways use different pressures but they generally range between 65 and 90 psi (4.5 to 6.2 bar).

A small pilot reservoir (the Equalising Reservoir) used to help the driver select the right pressure in the brake pipe when making an application.  When an application is made, moving the brake valve handle to the application position does not discharge the brake pipe directly, it lets air out of the equalising reservoir.  The equalising reservoir is connected to a relay valve (called the "equalising discharge valve" and not shown in my diagram) which detects the drop in pressure and automatically lets air escape from the brake pipe until the pressure in the pipe is the same as that in the equalising reservoir.

The equalising reservoir overcomes the difficulties which can result from a long brake pipe.  A long pipe will mean that small changes in pressure selected by the driver to get a low rate of braking will not be seen on his gauge until the change in pressure has stabilised along the whole train.  The equalising reservoir and associated relay valve allows the driver to select a brake pipe pressure without having to wait for the actual pressure to settle down along a long brake pipe before he gets an accurate reading.

The brake pipe pipe running the length of the train, which transmits the variations in pressure required to control the brake on each vehicle.  It is connected between vehicles by flexible hoses, which can be uncoupled to allow vehicles to be separated.   The use of the air system makes the brake "fail safe", i.e. loss of air in the brake pipe will cause the brake to apply.  Brake pipe pressure loss can be through a number of causes as follows:

A controlled reduction of pressure by the driver 

A rapid reduction by the driver using the emergency position on his brake valve 

A rapid reduction by the conductor (guard) who has an emergency valve at his position 

A rapid reduction by passengers (on some railways) using an emergency system to open a valve 

A rapid reduction through a burst pipe or hose 

A rapid reduction when the hoses part as a result of the train becoming parted or derailed. 

At the ends of each vehicle, "angle cocks" are provided to allow the ends of the brake pipe hoses to be sealed when the vehicle is uncoupled.  The cocks prevent the air being lost from the brake pipe.

The brake pipe is carried between adjacent vehicles through flexible hoses.  The hoses can be sealed at the outer ends of the train by closing the angle cocks.

Each vehicle has at least one brake cylinder.  Sometimes two or more are provided.   The movement of the piston contained inside the cylinder operates the brakes through links called "rigging".  The rigging applies the blocks to the wheels.  Some modern systems use disc brakes.  The piston inside the brake cylinder moves in accordance with the change in air pressure in the cylinder. 

The operation of the air brake on each vehicle relies on the difference in pressure between one side of the triple valve piston and the other.  In order to ensure there is always a source of air available to operate the brake, an "auxiliary reservoir" is connected to one side of the piston by way of the triple valve.  The flow of air into and out of the auxiliary reservoir is controlled by the triple valve.

This is the friction material which is pressed against the surface of the wheel tread by the upward movement of the brake cylinder piston. Often made of cast iron or some composition material, brake blocks are the main source of wear in the brake system and require regular inspection to see that they are changed when required.  Many modern braking systems use air operated disc brakes. These operate to the same principles as those used on road vehicles.

This is the system by which the movement of the brake cylinder piston transmits pressure to the brake blocks on each wheel.  Rigging can often be complex, especially under a passenger car with two blocks to each wheel, making a total of sixteen.  Rigging requires careful adjustment to ensure all the blocks operated from one cylinder provide an even rate of application to each wheel.  If you change one block, you have to check and adjust all the blocks on that axle.

The operation of the brake on each vehicle is controlled by the "triple valve", so called because it originally comprised three valves - a "slide valve", incorporating a "graduating valve" and a "regulating valve".  It also has functions - to release the brake, to apply it and to hold it at the current level of application.  The triple valve contains a slide valve which detects changes in the brake pipe pressure and rearranges the connections inside the valve accordingly.  It either:

recharges the auxiliary reservoir and opens the brake cylinder exhaust, 

closes the brake cylinder exhaust and allows the auxiliary reservoir air to feed into the brake cylinder 

or holds the air pressures in the auxiliary reservoir and brake cylinder at the current level.  

The triple valve is now usually replaced by a distributor - a more sophisticated version with built-in refinements like graduated release.

Operation on Each Vehicle 

Figure 3: Brake Release: This diagram shows the condition of the brake cylinder, triple valve and auxiliary reservoir in the brake release position.  Diagram: Author.

Brake Release: The driver has placed the brake valve in the "Release" position.  Pressure in the brake pipe is rising and enters the triple valve on each car, pushing the slide valve provided inside the triple valve to the left.  The movement of the slide valve allows a "feed groove" above it to open between the brake pipe and the auxiliary reservoir, and another connection below it to open between the brake cylinder and an exhaust port.  The feed groove allows brake pipe air pressure to enter the auxiliary reservoir and it will recharge it until its pressure is the same as that in the brake pipe.  At the same time, the connection at the bottom of the slide valve will allow any air pressure in the brake cylinder to escape through the exhaust port to atmosphere.  As the air escapes, the spring in the cylinder will push the piston back and cause the brake blocks to be removed from contact with the wheels.  The train brakes are now released and the auxiliary reservoirs are being replenished ready for another brake application.

Figure 4: Brake Application: The condition of the brake cylinder, triple valve and auxiliary reservoir in the brake application position. Diagram: Author.

Brake Application: The driver has placed the brake valve in the "Application" position.  This causes air pressure in the brake pipe to escape.  The loss of pressure is detected by the slide valve in the triple valve.  Because the pressure on one side (the brake pipe side) of the valve has fallen, the auxiliary reservoir pressure on the other side has pushed the valve (towards the right) so that the feed groove over the valve is closed.  The connection between the brake cylinder and the exhaust underneath the slide valve has also been closed.  At the same time a connection between the auxiliary reservoir and the brake cylinder has been opened.  Auxiliary reservoir air now feeds through into the brake cylinder.  The air pressure forces the piston to move against the spring pressure and causes the brake blocks to be applied to the wheels.  Air will continue to pass from the auxiliary reservoir to the brake cylinder until the pressure in both is equal.  This is the maximum pressure the brake cylinder will obtain and is equivalent to a full application.  To get a full application with a reasonable volume of air, the volume of the brake cylinder is usually about 40% of that of the auxiliary reservoir.

Figure 5: Lap: The purpose of the "Lap" position is to allow the brake rate to be held constant after a partial application has been made. Diagram: Author.

Lap: When the driver places the brake valve in the "Lap" position while air is escaping from the brake pipe, the escape is suspended.  The brake pipe pressure stops falling.  In each triple valve, the suspension of this loss of brake pipe pressure is detected by the slide valve because the auxiliary pressure on the opposite side continues to fall while the brake pipe pressure stops falling.  The slide valve therefore moves towards the auxiliary reservoir until the connection to the brake cylinder is closed off.  The slide valve is now half-way between its application and release positions and the air pressures are now is a state of balance between the auxiliary reservoir and the brake pipe.  The brake cylinder is held constant while the port connection in the triple valve remains closed.  The brake is "lapped".

In the traditional air brake with a triple valve, Lap does not work after a release has been initiated. Once the brake valve has been placed in the "Release" position, the slide valves will all be moved to enable the recharge of the auxiliary reservoirs.  Another application should not be made until sufficient time has been allowed for this recharge.  The length of time will depend on the amount of air used for the previous application and the length of the train. Modern air braking systems have a distributor in place of the triple valve. It performs basically the same function but also includes a graduated release capability and some other features that improve brake control.

Additional Features of the Air Brake

What we have seen so far is the basics of the air brake system.  Over the 130  years since its invention, there have been a number of improvements as described below.  A further description of the most sophisticated version of the pure air brake is available at my page North American Freight Train Brakes written by Al Krug.

Emergency Air Brake

Most air brake systems have an "Emergency" position on the driver's brake valve.  This position dumps the brake pipe air quickly.  Although the maximum amount of air which can be obtained in the brake cylinders does not vary on a standard air brake system, the rate of application is faster in "Emergency".  Some triple valves and most distributors are fitted with sensor valves which detect a sudden drop in brake pipe pressure and then locally drop brake pipe pressure. This has the effect of speeding up the drop in pressure along the train - it increases the "propagation rate".

Emergency Reservoirs

Some air brake systems use emergency reservoirs. These are provided on each car like the auxiliary reservoir and are recharged from the brake pipe in a similar way.  However, they are only used in an emergency, usually being triggered by the triple valve sensing a sudden drop in brake pipe pressure. A special version of the triple valve (a distributor) is required for cars fitted with emergency reservoirs.


As note above, a distributor performs the same function as the triple valve, it's just a more sophisticated version. Distributors have the ability to connect an emergency reservoir to the brake system on the vehicle and to recharge it. Distributors may also have a partial release facility, something not usually available with triple valves.

A modern distributor will have:

a quick service feature - where a small chamber inside the distributor is used to accept brake pipe air to assist in the transmission of pressure reduction down the train 

a reapplication feature - allowing the brake to be quickly re-applied after a partial release 

a graduated release feature - allowing a partial release followed by a holding of the lower application rate 

a connection for a variable load valve - allowing brake cylinder pressure to adjust autopmatically to the weight of the vehicle 

chokes (which can be changed) to allow variations in brake application and release times 

an inshot feature - to give an initial quick application to get the blocks on the wheels 

Figure 6: A modern distributor valve for use on freight cars. Photo: Wabtec.

brake cylinder pressure limiting 

auxiliary reservoir overcharging prevention. 

All of these features are achieved with no electrical control. The control systems comprise diaphragms and springs arranged in a series of complex valves and passages within the steel valve block. Distributors with all these features will normally be provided on passenger trains or specialist high-speed freight vehicles.

Two Pipe Systems

A problem with the design of the standard air brake is that it is possible to use up the air in the auxiliary reservoir more quickly than the brake pipe can recharge it. Many runaways have resulted from overuse of the air brake so that no auxiliary reservoir air is available for the much needed last application. Read Al Krug's paper North American Freight Train Brakes for a detailed description of how this happens.  The problem can be overcome with a two-pipe system as shown in the simplified diagram in Figure 7.

Figure 7: Schematic of a two-pipe brake system. The second pipe of the two-pipe system is the main reservoir pipe.  This is simply a supply pipe running the length of the train which is fed from the compressor and main reservoir.  It performs no control function but it is used to overcome the problem of critical loss of pressure in the auxiliary reservoirs on each car.  A connecting pipe, with a one-way valve, is provided between the main reservoir pipe and the auxiliary reservoir.  The one-way valve allows air from the main reservoir pipe to top up the auxiliary reservoir.  The one-way feature of the valve prevents a loss of auxiliary reservoir air if the main reservoir pressure is lost. Diagram: Author.

The two-pipe system has the ability to provide a quick release.  Because the recharging of the auxiliaries is done by the main reservoir pipe, the brake pipe pressure increase which signals a brake release is used just to trigger the brake release on each car, instead of having to supply the auxiliaries as well.

Two pipe systems invariably have distributors in place of triple valves.  One feature of the distributor is that it is designed to restrict the brake cylinder pressure so that, while enough air is available to provide a full brake application, there isn't so much that the brake cylinder pressure causes the blocks to lock the wheels and cause a skid.  This is an essential feature if the auxiliary reservoir is being topped up with main reservoir air, which is usually kept at a higher pressure than brake pipe air.

Needless to say, fitting a second pipe to every railway vehicle is an expensive business so it is always the aim of the brake equipment designer to allow backward compatibility - in much the same way as new computer programs are usually compatible with older versions.  Most vehicles fitted with distributors or two-pipe systems can be operated in trains with simple one-pipe systems and triple valves, subject to the correct set-up during train formation.

Self Lapping Brake Valves

Self lapping is the name given to a brake controller which is position sensitive, i.e. the amount of application depends on the position of the brake valve handle between full release and full application. The closer the brake handle is to full application, the greater the application achieved on the train. The brake valve is fitted with a pressure sensitive valve which allows a reduction in brake pipe pressure according to the position of the brake valve handle selected by the driver. This type of brake control is popular on passenger locomotives.

Other Air Operated Equipment

On an air braked train, the compressed air supply is used to provide power for certain other functions besides braking.  These include door operation, whistles/horns, traction equipment, pantograph operation and rail sanders.  For details, see Auxiliary Equipment.  


The air brake system is undoubtedly one of the most enduring features of railway technology.  It has lasted from its initial introduction in 1869 to the present day and in some places, still hardly different from its Victorian origins. There have been many improvements over the years but the skill required to control any train fitted with pure pneumatic brake control is still only acquired with long hours of practice and care at every stage of the operation.  It is often said that whilst it is easy to start a train, it can be very difficult to stop it.  Al Krug's paper North American Freight Train Brakes describes how difficult this can be.  Perhaps the trainman's skill is not quite dead yet. 

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