What Is a Train Braking System

A train’s braking system is vitally important to its operational safety in service; it is the sole system on the train responsible for ensuring the train stops. A train braking system is generally a pneumatic system meaning it functions by utilising compressed air. A braking system for a train is located on the underside of the carriage with the components either car body or bogie mounted. The major components of the pneumatic system are a compressor, air reservoir, pneumatic hoses, valves, brake holders and brake pads. It is generally operated by the driver when approaching a red signal or a platform by moving the brake handle to the desired position to give the desired braking force.

What Is A Train Braking System?

A train braking system is generally a pneumatic system located on the underframe of the car body. The role of the braking system is to slow the train down, which is down by converting the kinetic energy of the train to thermal energy.

How this is done on a train and many vehicles is by applying a force to the rotating brake discs bolted to the wheels or shrink fitted to the axle. The contact between the two surfaces converts kinetic energy into thermal energy.

The design of the braking system means that it is a fail-safe system. If there is a leak in the system, it will result in the brakes being applied. It is the change of pressure in the pneumatic piping that will result in the brakes being applied, held or released depending on the driver’s command.

Components of a Train Braking System

Compressor

The compressor has the role of compressing the air, ensuring that it is pressurised sufficiently to a suitable pressure. Older vehicles have reciprocating compressors, however, many modern trains have moved to screw or vane compressors, which have the additional benefit of reduced noise. The compressor pressurises enough air for all pneumatic systems on the train, including the pantograph, 3rd rail, pneumatic doors etc. as well as the brakes. 

For electric trains, compressors are driven by a motor, which they are directly coupled with. For diesel trains, a motor is also used in some instances, which is driven by the auxiliary alternator. On the other hand, compressors are also directly connected to the engine on some older fleets by a connecting shaft.

Using a motor to control the compressor has the benefit of being able to operate the compressor at different speeds depending on the requirements of the pneumatic system. When connected directly to the engine, the speed of the compressor can not be controlled independently, which may create some problems. 

As compressing air results in an increase in temperature, sets of cooling pipes are integrated into the compressor. However, the cooling will lead to condensation collection, which can clog the brake valves when mixed with debris and dirt. The primary method of overcoming this is using air dryers.

Governor

The governor is part of the control system for the compressor. When the air pressure has fallen below its lowest permitted level in the system, the governor contact switches close, sending a low voltage current to the compressor contactor. The signal closes the switch in the power supply for the compressor. Once the desired pressure has been reached, the governor opens, and the contactor switches off the compressor motor.

Pneumatic Piping

Pneumatic piping is generally installed along all coaches in a train. The role of the piping is to carry the compressed air to the desired location.   

Angle Cocks

Angle cocks are located at the end of each coach and are used to cut off the supply of compressed air. For example, for multiple units, the end coach will have its angle cock closed to prevent compressed air from leaving the system. 

Angle cocks are also used in a maintenance environment to cut off the supply of compressed air to allow maintenance activities to take place. 

Triple Valves 

A triple valve is commonly referred to as a distributor as it is responsible for distributing the air pressure to the required areas depending on the demand from the driver. It has connections to the brake pipes, auxiliary reservoir and brake cylinders. Additionally, it contains a path to the atmosphere to dispel pressurised air when required.

Air Reservoir

An air reservoir is a type of pressure vessel that stores compressed air ensuring there is sufficient supply when demand is required. Depending on the class of vehicle the number of air reservoirs will vary. However, it is normal practice to have one air reservoir per coach.

Brake Calipers 

The brake calipers are mounted onto brake hanger brackets and hold the brake pads. The brake calipers are closed and opened by the brake actuator.

Brake Actuators

Brake actuators actuate the brake calipers to give a compressive force on the brake discs. The actuators are pneumatically activated and directly connected to the brake calipers. 

Brake Pads 

Brake pads are contained within the brake calipers and are made from made of cast iron and composite materials. The brake pads convert the kinetic energy of the rotating brake disc into thermal energy when it compresses them.

Brake Discs

The brake discs are either bolted to the wheel or shrink fitted onto the axle. They have the same rotational speed as the axle, and hence slowing the brake discs will slow down the train.

How Are the Brakes In A Train Braking System Operated?

Application of Train Braking System

To apply the brakes, the driver moves the brake valve position to ‘Application’. Following the application, the air pressure in the brake pipes is expelled, resulting in a drop in air pressure in the brake pipes. The low pressure in the brake pipes means that there is a low-pressure region on the brake pipe side of the triple valve. On the other side of the triple valve, the auxiliary reservoir has maintained its high pressure resulting in a pressure differential across the valve. The pressure differential leads to the movement of the triple valve in the direction of the brake pipe.

The movement of the triple valve creates a direct path between the auxiliary reservoir and the brake actuator/cylinders whilst also blocking the feed grove to the brake pipe. The high-pressure input to the brake cylinders overcomes their springs and results in the application of the brakes. As the air enters the brake cylinder, the auxiliary reservoir loses pressure and continues until it is in equilibrium with the brake cylinder.

Release of Train Braking System

To release the brakes, the driver moves the brake valve position to ‘Release’. The air pressure in the brake pipe builds, leading to the triple valve’s movement towards the auxiliary reservoir. In doing so, the feed grove is opened, allowing the auxiliary reservoir to be pressurised to the same pressure as the brake pipes. 

The triple valve movement also exposes a direct link between the brake cylinder/actuator and the atmosphere. Once exposed, the air pressure in the brake cylinder dissipates and can no longer overcome the spring, leading to the brakes being released.