Train braking glossary

Have you heard the story of the browbeaten duck? He could fly as fast as the other ducks but he couldn't stop as quickly. To a train driver, the brakes are the most important part of the train but not many people outside the industry understand how they work. With that in mind, this page provides a simple glossary of brake equipment.

Detailed descriptions of different types of brake systems are available in

Early Brake SystemsAir BrakesVacuum BrakesE-P BrakingECP Brakes and North American Freight Train Brakes


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Air Brake

For details and graphics see the Air Brakes Page

This is the most common type of train brake.  It uses compressed air to apply the brake block (or pad) to the wheel and to control the operation of the brake along the train.  The compressed air is supplied by a motor driven compressor on the locomotive or train.

The brake control is actuated from a "driver's brake valve".  This valve is used to feed air to the brake pipe or to allow air to escape from the brake pipe.   A fall in brake pipe air pressure causes a brake application on each vehicle whilst a restoration of pressure causes the brake to release.

A distributor (or "triple valve" as it was always called and sometimes still is) on each vehicle monitors the pressure in the brake pipe.  When brake pipe pressure falls, the distributor allows air from an auxiliary reservoir on the vehicle to pass to the brake cylinders to apply the brake.  When brake pipe pressure rises, the distributor releases the air from the brake cylinder and recharges the auxiliary reservoir for the next application.  The release of air from the brake cylinder allows the block to be released from the wheel by a spring.

Air Dryer

A device provided on trains (usually next to the compressor) to automatically remove moisture from compressed air produced by the compressor.   If moisture is allowed to pass into pipework, it collects in valves and systems, reducing efficiency and causing rust.  Some older systems collected so much moisture than up to 20 gallons of water could be drained from a train.  To remove it, an old oil drum was wheeled under the train and the main reservoirs drained directly into it.

In days gone by, a main air reservoir under a vehicle could collect so much condensate (water) that a sharp frost could cause it to freeze and expand sufficiently to split the tank. See more under the Compressor description.

Analogue E-P Brake Control

A form of electro-pneumatic brake, normally restricted to multiple unit trains, which uses a single train wire to control the braking on each vehicle. The brake commands consist of pulses of electricity applied to the wire, a continuous signal denoting brake release and a loss of signal an emergency brake application. The brake control valve on each vehicle detects the length of the pulses and provides air input to the brake cylinders accordingly. The air supply is from the main reservoir pipe.

The analogue e-p brake system requires no brake pipe and the brake commands can be generated by a driver's brake controller or an automatic train driving system (ATO). It is also known as PWM (pulse width modulation) control or P-wire for short.

Angle Cock

A pneumatic isolating cock used on railway vehicles to shut off and/or drain air pipes (hoses) between vehicles. They are normally positioned at vehicle ends to allow the inter-connecting hoses to be isolated and, if provided with bleed holes, drained of air before being uncoupled. See also isolation.

Automatic Brake

The term is synonymous with continuous brake.

Auxiliary Reservoir

An air tank provided on each vehicle of a train equipped with air brakes to supply air for brake applications. More recently known as the brake reservoir.

Bar

Metric measurement of pressure equal to 14.5 pounds per square inch.

Bleed Hole

A small hole provided in the angle cocks of main reservoir hoses which opens when the angle cock is closed.  This has the effect of draining the air from the hose before it is uncoupled. Bleed holes are not provided on brake pipe angle cocks.

Brake Beam

A transverse member of the brake rigging which distributes the force from the brake cylinder to the brake blocks on either side of the wheelset.

Brake Blending

A system, used on modern, dynamically braked EMU vehicles and some locomotives, to ensure that air and dynamic braking acts in co-ordination.  An electronic signal from the electric (dynamic) brake indicating the brake effort achieved is compared with the brake effort demanded by the driver or an automatic control system and will then call up additional braking from the air brake system if required. See also dynamic braking.

A typical set-up on a car will comprise a brake control unit which contains electronic controls and electro-pneumatic valves. Various inputs are processed in the brake control unit which then generates electronic or pneumatic outputs as necessary.

Brake Demand: When a brake demand is requested by the driver (or the automatic driving control on an ATO equipped train) it is transmitted along a train wire to the brake control unit on each car.   The signal can be digital or analogue providing a message either in steps or infinitely variable.  The demand is then matched to a load compensation signal provided by the car suspension system.  The greater the weight, the greater the brake demanded.

The brake effort demand is now converted into air pressure signal and the brake is applied by sending air into the brake cylinders until it matches the signal.  At the same time, a matching demand is sent to the dynamic brake controller and the traction control system will initiate dynamic braking.  The system will send a "dynamic brake effort achieved" signal to the brake controller which will subtract it from the air brake demand signal and so reduce the brake cylinder pressure accordingly.

Dynamic Brake: In an ideal world, the dynamic brake will be used as much as possible to reduce wear on brake pads (or blocks) and there are often circumstances when the dynamic brake will provide all the braking required.  However, it is normal to leave a little air in the brake cylinders in case the dynamic brake switches off suddenly.   This reduces the time taken for air pressure to restore to the demand level when dynamic braking is lost.

Smoothing: Another feature of modern brake control is the "inshot".   This is a small amount of air injected into the brake cylinders immediately brake is called for so that the build-up time is reduced.  Braking systems are also "jerk limited"; smoothed out as they are built-up so that the passengers don't feel the cars snatch as the brake is applied. This is particularly important in the case of dynamic brakes which, if not jerk limited, have a tendency to apply sharply if the train is at speed.

Fade: Once the brake is applied, the dynamic portion will have a tendency to fade as the speed, and thus the current generated by the motors, reduces.  Some systems have a pre-fade control; a signal sent by the traction controller to indicate the brake is about to start fading.  This gives a smoother changeover into air braking.

Trailer Cars: Most types of EMU comprise a mixture of motor cars and trailers cars.   As trailer cars have no motors, they do not have their own dynamic braking.   They can, however, use dynamic braking on motor cars in their braking effort if that is available. In the case of a two-car pair, for dynamic demand, the motor car brake control unit will add the trailer car demand to the motor car demand.  The resulting dynamic brake achieved may be sufficient to match all of the motor car demand and have some extra for the trailer. In this case, the motor car brake control unit sends a message to the trailer car to say how much of the trailer car's demand has been fulfilled by the dynamic brake.  The trailer's air brake pressure can be reduced accordingly.

There will be some limit on the total dynamic brake possible because of adhesion limits and this will be incorporated into the brake control calculations.  If the dynamic brake is reduced for any reason, the trailer car air brakes will be reapplied first followed by the motor car brakes.

Brake Block

Material applied to the tread of the wheel tyre to effect braking on vehicles equipped with the tread brake system.  The block is hung from a lever or levers suspended between the brake cylinder and the wheel.  Cast iron was, and still is widely used but wood has also been used on some systems (e.g. Paris metro) and modern railways now use any of a wide variety of composition materials whose exact details remain the closely guarded secret of the suppliers. See also Brake Pad.

Brake Cylinder

The vehicle brake actuator used by both air and vacuum brake systems and consisting of a cylinder whose piston actuates the brake block lever.

Brake Frame

An assembly rack for train brake control equipment mounted under or inside a vehicle.   Sometimes referred to as a 'brake unit'.

Brake Pad

Composition material used as the friction medium on vehicles equipped with disc brakes. Brake pads for railway vehicles are similar to those used on road vehicles but larger. They are applied to the braking disc through levers operated by the brake cylinder. Such systems usually require a brake cylinder for each braking disc.

Brake Pipe

The pipe used to control train brakes on vehicles fitted with automatic air or vacuum brake systems.  In the US, often referred to as the 'train line'. On air braked trains, when charged, the brake pipe causes the train brakes to be released and the reservoirs (called auxiliary reservoirs) used to apply brakes to be automatically replenished.  When pressure in the brake pipe is reduced, train brakes are applied.

Brake Release Valve

A valve provided on each vehicle in a train to allow the brake to be released manually on that vehicle. Sometimes operated by a lever mounted in a suitable location for access by the crew or (on a suitably equipped EMU) can be operated remotely by the driver in the cab.  Some versions have a bleed hole on a brake isolating cock which performs the same function if it is necessary to isolate the brakes of one car from the rest of the train.

Brake Reservoir

Compressed air tank provided to supply air brake systems with air pressure for brake applications.  Modern systems usually require at least one brake reservoir under each vehicle.  Originally called the auxiliary reservoir and still often referred to as such.

Brake Resistor

This is a heat dissipating grid installed on a vehicle equipped with dynamic braking where the traction motors  are used as generators during braking.   The grids act much like an electric toaster, heating as the current is applied to them.  They absorb electrical energy generated by the traction motors acting as generators during braking and allow it to be transferred to the atmosphere as heat..   They can be mounted on the roof or under the locomotive or car.  Underfloor versions are sometimes fitted with fans (called blowers) to help get rid of the heat.

Brake Rigging

The means of distributing the braking forces from a brake cylinder to the various wheels on the vehicle.  It consists of rods and levers suspended from the underframe and bogies and linked with pins and bushes.  Rigging requires careful setting up and regular adjustment to ensure forces are evenly distributed to all wheels.  Badly set up rigging will cause wheel flats or inadequate brake force.

Brake rigging is now only found on older vehicles where there may only be one or two brake cylinders.  More modern systems usually employ one brake cylinder per one of two blocks or per disc.

Brake Systems

A competition to find a safe and reliable form of train braking held in 1875 at Newark, Lincolnshire, UK, showed two clear winners, the air brake, invented by George Westinghouse of the USA and the vacuum brake, of which there were then two examples.  Both required a pipe running the length of the train which was used to control the operation of the brakes on every vehicle.  Both were controlled from a valve on the locomotive.

The principle of the two systems was the same.  When the pipe was charged with compressed air or with a vacuum induced in it, the brake was released. When the pressure or vacuum was lost, the brake applied. Both systems used a cylinder on each vehicle which contained a piston connected to the brake shoes or blocks through rigging - a system of rods and levers.

The air brake was the clear winner in terms of stopping power and became widely used around the world but the vacuum brake was simpler and cheaper and was eventually adopted by most of the major railway companies in Britain.

The air brake was often called the Westinghouse Brake after its inventor even though many variations of it were and still are, built by other suppliers.

Brake, Types of

* the air brake, which uses compressed air to apply the brakes on each vehicle and as the driver's train brake control medium.

* the vacuum brake, which uses the atmospheric pressure in opposition to a specially created vacuum both to control and actuate the brake.

* the dynamic brake, which uses the electric motors of the traction power system to generate current during braking which is absorbed into a resistor (rheostatic braking) or back into the railway power supply (regenerative braking).

* the parking brake, used to hold an unattended vehicle when the braking system is shut down.  Often referred to as the 'handbrake' where it has to be manually applied on each vehicle as opposed to the automatic application provided on the most modern vehicles. Not all vehicles are equipped with parking brakes.

* the track brake, used on some light rail vehicles and trams where large magnets are hung under the vehicle over the rails and current is passed through them to induce a strong magnetic force.  The attraction between the magnets and the rails causes the vehicle to stop.  Mostly used for emergency braking.

Brake Unit

See brake frame.

Brake Van

A vehicle designed to allow a handbrake or the train brake to be operated by a person other than the driver.  Since, in the UK before the advent of continuous brakes, a guard (conductor) was provided to operate the brake on his vehicle to assist the driver stop the train, the "luggage van" was used and it became known as the brake van.  On freight trains, the same term was used for the vehicle used by the guard. In the US it is called a "caboose".

Passenger train brake vans were often combined with a passenger coach to form a "brake coach" as in "brake third" denoting a vehicle with a third class passenger section and a guard's position.

Clasp Brakes

A system of brake rigging where a brake block is applied to each side of a wheel tread.   In essence, the wheel is "clasped" by a pair of brake blocks. Sometimes referred to as "double-block" braking.  Normally such designs are arranged so that the two blocks are linked by the rigging and act together but some have individual brake cylinders for each block.  In such a case, a 4-wheeled bogie would have eight brake cylinders and a 6-wheeled bogie would have twelve brake cylinders.

Compressor

A motor driven pump mounted on a locomotive or train to supply compressed air for the operation of brakes and other pneumatic systems on the train.  Doors, whistles, traction control systems, automatic couplers and window wipers are all devices which can be operated by compressed air.

The air pressure is normally supplied in a range of between 90-110 and 130-140 psi. or roughly 7 - 10 bar (metric).  The operation of the compressor is usually automatic, being controlled by a pressure switch or "compressor governor".  The pressure switch switches on the compressor when air pressure falls to its lowest permitted level, say 90 psi and switches it off when it has reached its highest permitted level, say 110 psi.  At least one reservoir, called the Main Reservoir, is provided on the vehicle to store the compressed air.

Because compressed air produced by the compressor gets heated during the process, then cools afterwards, condensation occurs.  Eventually, water can collect in pipes and reservoirs. It often mixes with the compressor lubricating oil to form a sludge which gets into valves and prevents them working properly. In cold weather, it can freeze and split pipes or reservoirs.

Many compressors are designed to compress the air in two stages.  The air passes through cooling pipes after each stage to reduce the condensation and the second set of pipes are designed to allow the air to drain into the main reservoir.  There is a water trap and valve in the bottom of the main reservoir which automatically ejects excess water.  In many modern designs, an air dryer is provided between the compressor and the main reservoir.  The condensation is removed by the drying agent and ejected at the end of the compression cycle as the compressor governor switches off the compressor.

Compressors are normally provided on a locomotive or other vehicle where a power supply is available.  In a diesel locomotive the compressor may be driven directly off the engine or off the electrical supply generator.  Often, a small, battery-driven auxiliary compressor is provided as well to allow an air supply to be available for starting purposes, e.g. to allow a pantograph to be raised on a "dead" locomotive so it can get power.

A multiple unit train may have two or more compressors located under suitable cars which will supply air to the train through the main reservoir pipe.  The operation of the compressors will usually be synchronised via a control wire linked to the compressor governors so that they all operate in unison.

Continuous Brake

Generic term for a train brake which provides for control of the brake on every vehicle in the train and is automatic to emergency stop in the case of loss of control.  In other words, it is fail safe. In most countries it is a legal requirement for passenger trains.

The train will automatically stop if the train becomes uncoupled, if brake pipe is ruptured, if a brake valve is opened by passengers or staff and if the compressed air supply fails.

Note that some non-passenger trains do not always have all vehicles fitted with brakes.   Such vehicles are sometimes referred to as "swingers".

Digital E-P Brake Control

A development of electro-pneumatic (e-p) brake control is the digital control system.  It is normally only used on multiple unit trains. It incorporates the fail-safe features of the air brake but eliminated the need for a brake pipe.  The brake pipe is replaced by a "round the train wire" which is permanently energised.  As long as it remains energised, the brake remains released.  If it loses current for any reason, an emergency application follows.

Each car is equipped with a relay valve with can operate the brake in up to seven steps.  Three control wires are used in different combinations to actuate the seven steps of braking.  They are de-energised to apply the brake and energised to release.   Control is from the driver's brake handle in the cab or it can be by an automatic system such as ATO.

A well-known version of digital brake control is the Westcode system by Westinghouse.

Disc Brake

="Brake-Discs.jpg"

As used on trains, the disc brake (photo left) is similar to the disc brake used on road vehicles but may take the form of a pair of discs mounted either side of the wheel web or a double-sided self-ventilating disc mounted on the axle.  Very high speed trains, such as the French TGV, have up to four sets of double discs per axle.   The design and number of discs is critical to train safety as they must be capable of dissipating the maximum amount of heat generated during an emergency brake application from the highest speed attainable by the train.  Disc brakes on trains are invariably air operated.

Distributor

Air brake control valve (derived from and known as a triple valve on older systems) mounted on each vehicle which controls the passage of air between the auxiliary reservoir and the brake cylinder and between the brake cylinder and atmosphere.  The operation of the valve is controlled by changes of pressure in the brake pipe.  See also Air Brake.

Driver's Brake Valve

The means by which the train brakes are controlled.  On the classic air brake, the driver's brake valve has five positions: Release and Charging, Running, Lap, Application and Emergency.  In "Release and Charging" the brake pipe is supplied with air from the main reservoir and the pressure rises to release the brakes and recharge the auxiliary reservoirs.  In "Running", the brake remains released but a feed valve, attached to the driver's brake valve, is connected between the main reservoir supply and the brake pipe. This valve holds the brake pipe release pressure against any small leaks in the pipe.

The "Application" position drains air from the brake pipe to apply the brakes.  "Lap" is selected when the brake pipe air has fallen to the level required by the driver to give the application he wants.  In this position the connection between the brake pipe and the brake valve is closed.  In the "Emergency" the brake pipe air is dumped through a large opening in the valve so the air exhausts more quickly than with a normal application.  For more details, see Air Brakes.

The electro-pneumatic brake will also have a driver's brake valve if the air brake is provided as well.  Electrical connections are added to the operating spindle so that movement of the handle can operate either brake.  Later e-p systems with no brake pipe use what is called a "brake controller", which is simply an electrical controller to change the switch connections to the train control wires as required.

Dump Valve

An electrically controlled valve used to reduce air brake cylinder pressure in the event of wheel slide or skidding as part of a wheelslip control system.  Also the same type of valve is used to reduce air pressure in air suspension systems when the load on the vehicle is being reduced.

Duplex Gauge

An air gauge located in the driver's cab with two indications - main reservoir pressure and brake pipe pressure. Some railways also use a brake cylinder pressure gauge or gauges in the cab.

Dynamic Braking

A train brake system where the traction motors are used to provide a braking force by reconnecting them in such a way that they become generators.  Al Krug, referring to diesel-electric locomotive braking in a reply to a question in a newsgroup, put it this way (slightly edited by me):

Dynamic brakes are fundamentally no different from locomotive air brakes.  Both systems convert the energy of the rolling train into heat and then throw away that heat.   If you apply the loco air brakes, the brake shoes are pushed against the wheel treads and the resulting friction produces heat.  The energy required to produce this heat power makes the loco hard to keep moving.  The heat power is thrown away into the air by radiating from the hot brake shoes and hot wheel treads into the surrounding atmosphere.

A loco with air brakes applied is hard to keep moving but it will keep going, particularly if it has energy to move it in the form of a train pushing it from behind.   The energy (kinetic energy, it's called) comes from the rolling train that is pushing it.  The trouble with using engine brakes alone is that eventually (rather quickly actually) the shoes and wheels get very hot.  Hot enough to destroy them.   This is because heat is produced faster than it can be dissipated by radiating it into the air.  So dynamic brakes are used to move the heat dissipation away from the brake shoes and wheel treads to the dynamic brake grids instead.  Like an electric bathroom heater, the dynamic brake grids are designed to handle this amount of heat power (as long as the grid cooling blowers are operating).

Train air brakes work in the same manner as loco air brakes.  They convert the rolling energy of the train into heat and throw it away.  But when using train brakes, the heat generated is dispersed through out the entire train.  It is spread over (say) 800 wheels instead of just the few wheels of the loco.  Because of this, the train's wheels do not normally get overheated.  They will get warm or even hot but not normally so hot as to cause damage.  On prolonged downgrades, however, the braking energy required is sufficient to overwhelm the heat dissipating ability of even all the train's wheels and overheating occurs.  This is the main reason for using dynamic brakes, to move the heat dissipation away from the wheels to the dynamic brake grids.

Remember that it takes a 3,000 HP diesel engine just to turn the generator on a 3,000 HP loco.  Commercial generating power plants require 100s of thousands of HP to turn the generators that supply your household power.  Generators are hard to turn when they are producing power.  This is because you never get anything for free.  If you take power out of a generator you must put at least equal power into it. (Actually more than equal since nothing is ever 100% efficient either).

In locomotive dynamic brakes, the traction motors are acting as generators.  That means the traction motors are hard to turn. The loco's wheels are what are turning the traction motors.  They are geared to the traction motors.  This means the loco's wheels are hard to turn.  They resist turning because they are geared to the traction motors which are hard to turn when generating power, as they are doing when in dynamic braking.  Because the loco's wheels are hard to turn when in dynamic braking the loco is hard to move or in other words it resists movement just as if the airbrakes were applied making the wheels hard to turn.  The energy required to push this "hard to move" loco comes from the rolling train.  This removes energy from the rolling train slowing it.

Note that dynamic brakes are used by electric multiple unit trains as well. In these designs, careful blending with air braking is required to maintain a smooth braking profile.  Electronic control is used to determine that the brake effort demanded by the brake controller is matched by the brake effort achieved by the train.   Preference is given to the dynamic brake to save wear on brake blocks (shoes) or pads and air braking is added if necessary to achieve the braking rate required.

Dynamic braking can be used on electric railways to convert the energy of the train back into usable power by diverting the braking current into the current rail or overhead line.  This is known as regenerative braking.  It is used in the same way as rheostatic braking but the energy can be used by other trains requiring power.  The power developed by a braking train may not be accepted by the line if no other trains are drawing power so trains equipped with regenerative braking will usually have resistor grids as well to absorb the excess energy.  The balance between regenerated current and rheostatic current is also controlled electronical. See also our Electric Traction Pages Page and under Dynamic Brakes in North American Freight Train Brakes.

EOT Device

An EOT (End Of Train) device is mounted at the rear of a US freight train and is triggered to open a valve on the brake pipe when an emergency application is called for by the driver.  A cab unit has a covered switch which, when activated by the driver, sends a radio signal to the EOT. Two-way digital encoding ensures that only the locomotive on the particular train is capable of activating the valve. The device is battery-powered and provides the train with the rear end red light as well. The system is a legal requirement on US railroads and was instituted over the last couple of years following cases where angle cocks between cars had been closed (in some cases maliciously), rendering those cars remote from the locomotive brakeless.

EOTs are also used to provide an indication that the brake pipe of the train is complete by sending a signal back to the driver's cab when there is a change in pressure.

Early Brake Systems

Originally, the only way to stop a train was by applying a brake to the wheels of the locomotive.  A wooden block was applied to the wheel tread.  A lever operated by the driver actuated the brake.  If more brake power was required, the driver reversed the engine as well.  Soon however, it became apparent the this was not enough to bring the train to a stand in a reasonable distance and anyway, the reversing of the wheels damaged the wheel treads, so various vehicles in the train had brakes added.   The brake was hand operated by a lever or screw arrangement, so a man was appointed to ride on each of these "brake vans" as they were called.  As trains became heavier and faster, more brake power was required and more brake vans were added.

The principal disadvantages of the manual braking system were that it required additional staff along the train and there was little co-ordination during braking.   The driver used the engine to whistle for brakes and to signal for release.

ECP Braking

See ECP brakes page.

Electro-Pneumatic Brakes

For details and graphics see the E-P Brakes Page

The traditional air brake works well enough in the hands of a skilled driver but it has a number of shortcomings.  Its control system relies on the changes in brake pipe pressure to control the application and release of the brakes.  This means that a command by the driver to alter the pressure is felt by the front of the train first and then gradually by the rest of the train until it reaches the end.  This can cause trouble on a long train if it is not handled carefully, particularly during release when leading vehicles in release mode can pull on rearmost vehicles which still have brakes applied.

The brake pipe is also used to replenish the air reservoirs on each vehicle, a slow process on a long train.  Time has to be allowed between successive applications for reservoirs to recharge.  Finally, the automatic air brake has no partial release capability.  Once the driver has demanded a release, it will happen and brakes can only be reapplied when the reservoir pressure has recharged to a value higher than the brake cylinder pressure.

What was recognised many years ago was that electrical control could overcome these problems.  Since the early 1900s, when electrical control of brakes was tried on the New York Subway, various systems and solutions have been tried.

Most electro-pneumatic brake systems have been designed so that they can be added to the traditional air brake system to allow more rapid responses to the driver's braking commands.  For example, in the simple version used on the UK High Speed trains, each end of the train has an electrically operated valve.  When an application is called for at one end, the valve opens the brake pipe at the other end so that both ends are exhausting air at the same time.  A simple version of this, called an EOT (End of Train device) is used on US freight trains for emergency application.

A earlier development first tried on multiple unit trains in the UK in the 1920s, consisted of a system whereby the application and release of the brake was achieved by electrically controlled valves on each vehicle. It was originally designed early this century for rapid transit trains in the US to overcome the natural delay which occurs due to the propagation of the pure air brake and quickly adopted in Europe. Normally the electrical control is additional to and superimposed upon the automatic air brake, although more recent systems incorporate a fail safe electrical control which eliminates the need for a separate brake pipe. See digital and analogue e-p systems.

A basic e-p brake system as applied to a multiple unit train comprises an electrically operated "holding valve" and "application valve" on each car together with control wires running the length of the train. The main reservoir is also connected to each car on the train by a main reservoir pipe. Often more than one main reservoir is provided. Usually, each car also has an "e-p brake reservoir".

The e-p brake operates independently of the air brake. It uses main reservoir air instead of brake pipe air and the air brake and triple valves are kept in the release position. The e-p brake is controlled from the same driver's brake valve as the air brake but using new positions to apply and release the e-p brake. Electrical connections attached to the driver's brake valve send commands along the train to the holding and application valves on each car.

To apply the brake the driver selects "Application", which causes all holding and application valves to energise. The holding valve closes off the brake cylinder exhaust and the application valve opens to admit main reservoir air into the brake cylinder. The brakes apply. Selecting "Release" de-energises the valves, closing the application valve and cutting off the main reservoir pipe connection and opening the holding valve to allow brake cylinder air to exhaust.

The advantage of the e-p system is that it allows instantaneous reaction on all cars at the same time and it allows small and graduated applications and releases. This gives accurate and rapid stopping, which is particularly important in suburban and rapid transit operations.

E-P brakes are not normally used on freight trains because of the diversity of wagons and the cost of conversion. Also, getting an electric signal to transmit at a low voltage down a very long train is difficult. Radio control has been suggested, as has fitting each car with a battery. Some experimental e-p systems are being tried in the US in an attempt to improve brake control. The real test however, will be the willingness of railway companies to spend time and money doing the conversions.

Empty/Load Lever

A device for varying braking on freight cars so that braking is adjusted in accordance with the weight of the vehicle. It its usual form, a lever at the side of the wagon has two positions, "Empty" and "Loaded". Changing the position of the lever adjusts the brake rigging so that the brake force is adjusted to compensate for the empty or loaded condition of the vehicle.

Equalising Reservoir

Air reservoir employed in air braking systems to provide the driver with brake control valve with a greater degree of flexibility and to create a cushion for brake pipe control between the driver's manually operated brake valve and the brake pipe. For more details see North American Freight Train Brakes.

Exhauster

A pump, usually electrically driven, which removes air from the brake pipe of a train equipped with the vacuum brake. Equivalent to the compressor on an air brake system. Performs the same function as the ejector on a steam locomotive. Exhausters are usually designed to run at two speeds - slow speed to maintain the vacuum against small leaks and losses along the brake pipe and high speed to get brakes released after they have been applied.

Feed Valve

A pressure regulating valve provided in the driver's cab to allow the brake pipe pressure to be held at a constant level while the train is running with the brake released. Some railways, notably those in the US, allow this valve to be adjusted by the crew. See also Driver's Brake Valve and North American Freight Train Brakes.

Flats

A damage spot on a wheel tread caused by the wheel locking and skidding during braking. The skidding is caused by reduced adhesion between wheel and rail and it will extend the braking distance required for a given brake application. The flats will be heard as the train restarts and will continue until the wheel treads are reprofiled in the workshop. Severe flats are considered dangerous as they may cause derailments at points so they can cause a train to be removed prematurely from service.

The problem of flats has become worse as passenger rolling stock, particularly multiple unit trains, has tended to become lighter, thus reducing the adhesive weight. Further problems have developed with the trend towards disc brakes instead of tread brakes. At least with a tread brake, the action of the block rubbing against the wheel had a scrubbing effect on the surface and helped keep it clean.

In many countries where there is a leaf fall season, the effect of crushed leaves on rails has caused significant problems with adhesion.  Some lines are forced to introduce temporary speed restrictions and, in London, England, a special leaf fall timetable was used on one line where times were increased to compensate for longer braking times at stations.

Foundation Brake Gear

Another term to denote brake rigging which distributes braking forces around to the wheels of a vehicle having only one or two brake cylinders.

Friction Brake

The most common type of brake used by trains, it acts by dissipating the kinetic energy of the moving train by converting the energy into heat.  The heat arises from the friction between the brake pad and the brake disc or between block and wheel tyre during braking.  See also Dynamic Brake.

Graduated Release

One drawback of the basic air brake system is that there is no way of gradually releasing the brakes.  To release the brakes, the driver will recharge the brake pipe using the "driver's brake valve". Once the air pressure in the brake pipe start to build up, the triple valves detect this rise in pressure and move to the release position and exhaust brake cylinder air.

Handbrake

Nowadays synonymous with the term parking brake but originally a vehicle brake applied by hand action to a wheel or lever on the vehicle.

Hose

Braking and other systems on the train use compressed air both as a power source and a control medium.  The connections between vehicles are through flexible pipes usually referred to as hoses.  Hoses are normally equipped with isolating cocks to shut off the supply and bleed the hose when vehicles are uncoupled.

Isolation

Under various conditions, it is necessary to isolate portions of or the whole brake system.  For this purpose, isolating cocks are positioned in suitable locations.   The most obvious isolating cocks are called angle cocks and these are used to allow vehicles to be pneumatically isolated when uncoupled.

Another isolating cock is provided in the pipe connecting the main reservoir to the driver's brake valve. It is important to ensure that this cock is closed on any locomotive where the brake control is not being used so that air does not get into the brake pipe while the driver is trying to apply the brake from another driving position.

On e-p brake equipped vehicles, it is common to allow the equipment to be pneumatically isolated so that the air brake can be used instead.

Various isolating cocks with bleed holes are fitted to allow reservoirs to be drained so that equipment can be worked on safely or reservoirs can be drained of water which appears from condensation.

Main Reservoir

Compressed air storage tank provided on trains to supply pneumatic systems including brakes. See Compressor for details.

Main Reservoir Pipe

An air pipe provided on multiple-unit trains to supply pneumatic equipment located along the train such as doors, brakes etc. connecting all main reservoirs on a train from which supplies for pneumatic systems are drawn. Connections between cars are via flexible hoses. Normally, each vehicle has main reservoir pipe isolating cocks at each end of the pipe to allow uncoupling of hoses without loss of main reservoir pipe pressure.

One Pipe/Two Pipe Systems

The main disadvantage of a train braking system using an air pipe to control the brakes is the propagation time - the time taken for a change in pressure to reach all the vehicles. The longer the train, the greater is this time. Some long US freight trains may take as long as 15 minutes to recharge a completely airless brake system. Even on a short train, with a fully charged brake system, it may take several seconds for the rear vehicle to respond to changes in brake pipe pressure.

One way of reducing the recharge time and getting a quicker release of brakes is to use a second pipe. The second pipe is the main reservoir pipe, which is recharged directly from the compressor. It is constantly kept at full pressure, regardless of the status of the brake pipe pressure. When brake release is selected, the distributors on each vehicle use this main reservoir air to recharge the auxiliary reservoirs instead of using brake pipe air as on the one pipe system. During an application, some systems add main reservoir air to the brake cylinders to speed up operation.

The two-pipe systems is also a feature of some electro-pneumatic brake systems.

P-Wire Control

A short form of "Pulse Width Modulation brake control".  A type of electro-pneumatic brake control; see Analogue E-P Brake.

Parking Brake

Means by which an unattended or unpowered vehicle can be secured against unplanned movement. Usually consists of a manually applied friction brake applied to the wheel tread or disc. Some recent developments include spring-applied parking brakes, which release when compressed air for the air brake is available, and pneumatically applied systems.

Pinning Down the Brakes

A term used in the days of freight train operation when some or all the wagons in a freight train had no continuous brake, only a hand brake.  The hand brake was operated by a lever on the outside of the wagon and held down in the application position by a pin in a hole.  Freight trains about to descend a gradient would stop and the guard would "pin down the brakes" on some vehicles to control the speed.   At the bottom of the gradient, the train would stop again and the brakes would be released manually.

Propagation Rate

The rate at which the change in air pressure initiated by the operation of the driver's brake valve travels along the brake pipe.  Because it takes time for the reduction (or increase) in pressure to travel along the brake pipe, the brake applies (or releases) at different times on different vehicles.  This affects the control of the train and can cause bunching or stretching of vehicles putting additional strain on the couplings, particularly on long, heavy trains.  Skill is required by the driver of a heavy freight train to ensure that operation of the brakes does not cause a train to break apart or even derail.

Regenerative Braking

A braking system used by locomotives and trains fitted with electric traction motors where the motors become generators and the current developed is fed into the overhead line or third rail for use by other trains or for return to the supplier.  For more details, see Dynamic Braking.

Retainer

A manually operated valve mounted on many US freight cars to provide a constant minimum application even though the brake has been released from the driver's brake valve in the cab.  Normally, when brakes are released, all of the air in the brake cylinders is discharged to the atmosphere.  By setting retainer valves, when the brakes are released, some of the air pressure is "retained", hence the name.  If set for say 10 psi, the brake cylinder pressure will not drop below 10 psi until the retainer is reset (or until the air eventually seeps out).  Typically, a certain number of cars on the rear of the train would have their retainers set by the conductor.   Rulebooks indicate how many retainers to set before descending.

There are 2 types of retainer valves, a 3 position type and a 4 position type.   The operating positions are:

  • EX- Exhaust, normal will not retain air
  • HP-High Pressure, will retain 20psi
  • SD-Slow Direct Exhaust, will not retain air but will exhaust the air more slowly then normal
  • LP-Low Pressure, will retain 10psi, only available on the 4 position retainer valve.

If you make a stop on a grade and have to release the air brakes to recharge the system, then before you release the air brakes you must apply a sufficient number of hand brakes to secure the train if you are not sure the locomotive brakes will hold the train.   Depending on conditions, that may mean every hand brake on the train.  (I know I would not want to be the conductor who had to set 100 hand brakes.)  After that job is completed then the air brakes can be released and the system may be recharged. If you do attempt to use the locomotive brakes to hold the train and it turns out that they are not sufficient to hold the train you may not have enough air left to stop the train again.

Written with newsgroup contributions from David Gianna and Donald Reventlow.  See more in North American Freight Train Brakes.

Rheostatic Braking

A braking system used by locomotives and trains fitted with electric traction motors where the motors become generators and the current developed is fed to on-board resistor grids.  The energy is dissipated as heat as the grids cool.  Some grids have to be force-ventilated to dispose of the heat quickly enough.  Rheostatic braking is useful for diesel-electric locomotives with heavy freight trains on long down grades.   For more details, see Dynamic Braking.

Self-Lapping Brake Valve

A type of driver's brake valve where the position of the valve operating handle between "brake off" and "full application" determines the rate or level of brake application. This type of brake valve can be seen on some air braked and e-p braked trains.

Slack Adjuster

Most modern rail vehicles have brake cylinders equipped with slack adjusters. The slack adjuster automatically compensates for the wear induced in the block or pad during braking. It operates usually with some form of ratchet system fitted internally or as part of the brake cylinder assembly.

Slip/Slide Control

A critical feature of the railway environment is the interface between wheel and rail. This interface is dependent upon the adhesion between the steel surface of the wheel tread and the steel surface of the rail head. The relationship is defined as the coefficient of friction. On a dry day is this is about 0.3, on a wet day 0.2 with clean rails. A figure of 0.1 is allowed for normal braking and 50% of that added as a safety margin to prevent overrunning. Values under 0.05 will occur in conditions where the rail head is contaminated by leaves or ice.

The coefficient of friction figures relate to circumstances where there is no sliding action between wheel and rail. Tests have shown that braking distances will increase considerably if the wheels slide during braking. There is nothing worse for a driver who applies the brake and then sees the speedometer drop instantly to zero. He knows he will not be able to stop in the right place. There will also be wheel damage, called flats.

Detection: In order to reduce the likelihood of excessive braking, many locomotives and multiple units are fitted with wheel slip/slide control systems. The most common of these operates rather like ABS (automatic braking systems) on road vehicles. The railway systems usually monitor the rotation of each axle and compare rotational speeds between pairs of axles. If a difference appears between a pair of axles during braking, the brake is released on those axles until the speeds equalise, when the brake is re-applied. All this occurs automatically. Modern systems also detect too rapid deceleration of an axle.   Another form of slip/slide detection uses Doppler radar techniques.  This measures the ground speed of the locomotive against the revolutions of each wheelset and uses the detection of a difference to regulate the control systems.

Practice: The purpose of a wheel slide protection system is to reduce wheel damage. If anyone was to suggest that it should allow the train to stop safely within the normal braking distance for a given degree of application, they would be wrong. What slip/slide control can do is to regulate the braking to within 10-20% of the best available adhesion. Nevertheless, in practice, the effect of poor railhead conditions lengthens the actual braking distance over that normally required with good rail conditions. To a driver, this makes his braking distance longer because he will normally start braking according to position and not according to speed.

A section of line over which a driver passes often will allow him to determine over time that the best point to commence braking in order to stop at, say, a station is in a particular position, using a landmark - tree, signal post, bridge or something similar. This assumes that the train speed is usually the same each time he passes this point.

The theoretical concept for slip/slide control would only be valid if the driver knew in advance that the wheel rail adhesion would be reduced and made a normal application of the brakes in advance of the usual braking commencement point. Because of the reduced adhesion, the normal application would induce the wheel slide control and, instead of stopping short of the correct position as he would have done with a dry rail, the train will stop in the correct position under the control of the wheel slide protection system.  

ATO: Wheel slide control has further limitations when in use on an automatic train operation (ATO) system. On suburban commuter lines, subways and metros, many of which use ATO systems, rapid braking is necessary to reduce the headway and the train control system is designed to do just this. It would require some pretty sophisticated detection systems to alert the ATO to poor adhesion if wheel slide was to be automatically controlled and safe braking distances adhered to. An accident on the Washington subway system a few years ago was caused by a train sliding on iced rails whilst braking into a station, failing to stop in the correct distance and hitting a parked train beyond the station. Most ATO systems used on open lines have additional margins built into the braking control to compensate for poor adhesion.

In many instances, the wheel slide control is combined with wheel slip control. Wheel slip occurs during acceleration and is therefore not part of the braking system. It has however, become pretty sophisticated with creep control allowing good acceleration with virtually no equipment damage.

Spring Applied Parking Brake

A system for automatically applying a parking brake to a vehicle when the automatic air brake pressure is not available. Manually operated parking brakes can be forgotten by the crew - forgetting to apply them (or enough of them) when stabling a train or forgetting to release them before moving a train. The latter is a common problem which causes wheels to be dragged and damaged by flats. The spring applied parking brake attempts to overcome these problems.

The principle of operation is that the brake is held released by air pressure and is applied by a spring when the pressure is lost. It acts in the opposite way to the air brake.  There are also remotely applied spring parking brake systems available, which can be activated from a push button in the driver's cab.  Quite why this complication is necessary escapes me but some railway administrations insist on it.

Straight Air Brake

A simple compressed air brake fitted to locomotives for use on the locomotive only. It comprises an air supply (compressor), a driver's brake valve and connections to the brake cylinders. There is no automatic safety feature as is provided on automatic train brakes. The driver's straight air brake valve is normally provided separately from the train brake valve and is operated independently from it. However, use of the train brake will operate the locomotive brakes, unless the driver operates a special isolating feature. For details see North American Freight Train Brakes.

Train Brakes

During the early 19th century various attempts were made to get away from the concept of vehicle brakes which had to be individually controlled and provide a train brake with one point of control. A scheme of 1840 had a chain which ran along the train to the guard's position at the rear where it was wound round a drum. To apply the brake the drum was lowered until it touched an axle, causing it to rotate and tighten the chain. Levers connected to the chain applied the brakes. Variations of this idea all suffered from the problem of breakages and the effects on the brake chain caused by the compressing of the couplings between vehicles.

In addition, as railways developed during the mid 19th century, there were a number of accidents caused by trains becoming uncoupled (a breakaway) or just failing to stop. Sometimes, breakaways ran down a grade and collided with the following train or trains became parted and the second half ran into the front half after the crew had stopped it because they had noticed the uncoupling.

Some form of safeguard against these problems was needed and various ideas were put forward to provide brakes on every vehicle (so-called continuous brakes) and to control them from the locomotive. Various methods were tried, including ropes, chains and pipes running along the length of the train until it was decided to hold a competition at Newark, Lincolnshire in the UK in 1875 to find the best practical solution.

Train Line

In UK parlance, a cable running the length of a train for any train control or power purposes. The term train line is sometimes used in the US and on London Underground to denote the brake pipe.

Tread Brake

The traditional form of wheel brake consisting of a block of friction material (which could be cast iron, wood or nowadays a composition material) hung from a lever and being pressed against the wheel tread by air pressure (in the air brake) or atmospheric pressure in the case of the vacuum brake. Now the preferred system is a disc brake which, by replacing the tread brake, removes one of the causes of wear on wheel treads, although it also removes the scrubbing action which tended to reduce the risk of wheel slide.

Triple Valve

The principal control valve mounted on each vehicle fitted with air braking. So-called because it has three functions - to apply the brake, to hold the application at a constant level and to release and recharge the brake system. It also has (in its original form) three connections - to the brake pipe, to the brake cylinder and to the auxiliary reservoir.

Later versions of the original triple valve had a "quick acting" function brought into operation upon an emergency application or rapid discharge of brake pipe air. Recent developments have seen the original metallic valves replaced by flexible diaphragms and additional features like "quick release", "graduated release" and connections to emergency reservoirs added.

Triple Valve Operation

The triple valve operates by detecting differences in air pressure between the brake pipe, the brake cylinder and the auxiliary reservoir. To see how it works in detail, try this link at our page on Air Brakes.   This provides a useful description of the air brake control system as it is used world wide.

Vacuum Brake

For details and graphics see the Vacuum Brakes Page.

The automatic braking system where the brakes on each vehicle are actuated by the action of atmospheric pressure over a pre-formed vacuum. The brake pipe- is normally evacuated by a motor driven exhauster to create a vacuum and release the train brakes.

The vacuum developed in the brake pipe is measured in inches of mercury and is usually in the range of 21 to 25 inches for a fully charged system. As the degree of vacuum was by no means standardised in the UK, this caused some problems on joint services. For example, cases are recorded where Southern Railway engines could not release the brakes on Great Western Railway trains because the GWR vacuum was higher and the SR engines could not create sufficient vacuum to equalise throughout the train. They had to destroy the vacuum completely and start again - this causing delays.

The system has fewer valves than the equivalent air brake system but it has the disadvantage that response time and braking distances for a given weight of train are usually longer - over 50% in most cases.

In an attempt to speed up the propagation rate, later versions were fitted with accelerator valves on each vehicle. As soon as this valve detected a reduction in vacuum level, it admitted air locally into the brake pipe and brake cylinder, thereby speeding up the application. As soon as vacuum pressure was restored, the valve closed to prevent further air intake.

One big advantage of the vacuum brake is the ability to graduate release as well as application. The air brake triple valve was designed to allow a graduated application but, once set in the release position, it could not stop the release until the air pressure in the auxiliary reservoir was restored. Modern air braking systems are designed to overcome this and allow graduated release.

The vacuum brake is obsolete as far as railway braking is concerned but it is still used by those older equipped lines around the world which were based on British practice. For example, there are still EMUs operating in South Africa with vacuum brakes.

Variable Load Valve

Also known as a Retainer. A valve used to vary brake application individually on each vehicle depending on the weight of the vehicle, it can be manually or automatically operated. In the manual version - used on freight vehicles only - a lever at the side of the wagon must be set for the required position. For details of the system used in the US see Retainers.

Automatic versions of variable load valves are now often used. A simple version is operated by a lever connected between the valve mounted on the car underframe and the bogie frame. As the car load increases, the lever detects the depression of the car body and valve relative to the bogie and adjusts the setting of the valve in direct proportion. See also North American freight Train Brakes - Empty/Load Sensors.

On vehicles with air suspension, the lever is used to adjust a levelling valve which changes the air pressure in the suspension system so that the car body maintains a constant height, regardless of the load. Changes in the suspension air pressure are detected by a separate variable load valve and the brake application adjusted to suit.

The variable load system can also be used to adjust train acceleration so that it is constant regardless of load.

Westcode Brake

Proprietary train braking system by the Westinghouse Co. of the UK using a 7-step relay valve on each vehicle controlled by three train wires. It has a digital control system, with an additional round-the-train-wire designed to replace the brake pipe and energised-to-release control wires to give electro-pneumatic brake control. The control is fail-safe by causing an emergency brake application if either the signal on the train wires or the electrical supply is lost.

Wheel Slide

See Slip/Slide Control

More Information

Train Brakes FAQ - a page describing US air brakes from the San Diego R.R. Museum web site, written by Bob MacDowell.