Article and images by Piers
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 is a simplified description of the air brake
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".
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 Principal Parts of the Air
The pump which draws air from the atmosphere and compresses it for
use on the train. Its principal use is for the air brake system,
although compressed air has a number of other uses on trains. See
Storage tank for compressed air for braking and other pneumatic
Driver's Brake Valve
The means by which the driver controls the brake. 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.
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
"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.
"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).
This is a small pilot 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 equalizing reservoir. The equalizing reservoir is connected
to a relay valve (called the "equalizing 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 equalizing reservoir.
The equalizing 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 stabilized
along the whole train. The equalizing 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 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
- 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
- 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
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.
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 has three 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.
Operation on Each Vehicle
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.
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.
The purpose of the "Lap" position is to allow
the brake rate to be held constant after a partial application has
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".
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.
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
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 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".
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
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 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
- 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
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.
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.
Another advantage of the two-pipe system is its 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 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
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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