Auxiliary Systems on Rolling Stock


This page describes the on-board, compressed air auxiliary services required by trains and how they are provided on the locomotive and passenger vehicles.  A separate page is provided for Electrical Auxiliaries

See also the Rolling Stock Pages Links PageMultiple Unit OperationElectronic Power, Electric Traction Pages Drives, Electric Traction Pages (DC Resistance Control) and Electric Traction Glossary pages.


On-Board Services - Air Equipment - The Compressor - The Pump - Drives - Cooling - Drying - Control - Synchronisation - Storage - Distribution - Angle Cocks - Automatic Couplers - Air Operated Equipment - Traction Equipment - Doors - Air Suspension - Driver's Brake Control - Pantograph Compressor

On-Board Services

The modern passenger train provides a number of on-board services, both for passengers and control systems.  They are almost all electrically powered, although some require compressed air and a few designs use hydraulic fluid.  Since a train is virtually a self contained unit, all the services are powered and used on board.  Their use and features can be summarised as follows:

Compressed air - almost always used for brakes and sometimes for powering automatic doors.  Also once popular for powering traction power switches or contactors.  Usually used for raising pantographs on overhead line systems. Always needs drying after compression to avoid moisture from condensation getting into valves.  The compressor is normally driven directly from the main power source (the overhead line or third rail on electrified lines or the main generator on diesel powered vehicles).

Battery - Normally provided on locomotives and trains as a basic, low voltage standby current supply source and for start up purposes when livening up a dead vehicle.  The battery is normally charged from the on-board auxiliary power supply.

Generator - the traditional source on a train for on-board, low voltage supplies.  The generator is a DC machine driven by the diesel engine or, on electric locomotives, by a motor powered from the traction current supply.  On a coach, the generator was often driven directly off an axle (a dynamo), a large bank of batteries providing power for lighting when the train was stationary.

Alternator - the replacement for the generator which provides AC voltages instead of DC for auxiliary supplies.  AC is better than DC because it is easier to transmit throughout a train, needing smaller cables and suffering reduced losses.  Needs a rectifier to convert the AC for the battery charging and any other DC circuits.

Converter - the replacement for the alternator.  This is a solid state version of the alternator for auxiliary current supplies and can be a rectifier to convert AC to DC or an inverter to convert DC to AC.  Both are used according to local requirements and some designs employ both on the same train.  The name converter has become generic to cover both types of current conversion.

The rest of this page deals with compressed air supplies.  For details of electrical auxiliary equipment, go here.

Air Equipment

comp-02.gif (6872 bytes) Looking at compressed air supplies first, the diagram left (click for full size view) shows a typical arrangement for compressed air supply on a locomotive. The main items of equipment are a compressor, cooling pipes, an air dryer, a storage reservoir and controls.

The Compressor

The compressor itself consists of a pump driven by an electric motor. Power from the motor comes from the on-board electrical supply or, sometimes, directly from the traction supply. On electric locomotives, the supply can come from the transformer, via a rectifier and on a diesel locomotive, from the auxiliary alternator.  On some diesel locomotives, the compressor is driven directly from the diesel engine by way of a connecting shaft.

The Pump

The traditional compressor pump was a piston in a cylinder. Later, two or three pistons were provided to increase compression speeds and give greater capacity. Some compressor manufacturers offer rotary pumps, which are generally much faster and considerably quieter than reciprocating pumps. They are however, usually more susceptible to mechanical faults and have lower capacity than reciprocating pumps. Development of quiet, reliable compressors continues.

Useful descriptions of the various types of air compressor are available on the Compair page here.


Most compressors are directly coupled to their power source - usually the electric motor. Some are belt driven, another attempt to get quieter operation. Belt drives were particularly common on the continent of Europe. As mentioned above, some diesel locomotives drive the compressor pump directly through a mechanical link with the diesel engine, so there is no separate electric motor.


Compressing air makes it hot, so at least one set of cooling pipes will be provided. Some compressors have two sets. The pumping is split into two stages and a set of cooling pipes is provided between each, an inter-cooler and an after-cooler. Of course, the cooling produces condensation, which collects as water in the air pipes and, combined with oil from the compressor lubrication, forms a sludge which can quickly clog up sensitive brake valves. To overcome this problem, air systems are nowadays always provided with air dryers.


The air dryer consists of a pair of cylinders containing desiccant, which extracts the water and allows dry air to pass into the main reservoir. Water collected is automatically dumped once in each pumping cycle - the noise of the burst of water being discharged can often be heard at the end of the compressor's pumping cycle.


The compressor is controlled automatically by a "compressor governor". The governor is designed to detect the point at which the compressed air level in the system has fallen to the lowest permitted level. As this happens, the governor switch contacts close and send a low voltage (LV) current to a "compressor contactor". The contactor is energised and closes a switch in the power supply to start the compressor motor. When the pressure reaches the required upper limit, the governor opens and the contactor switches out the compressor motor.  All compressors also have an ON/OFF switch in the cab and there is usually a way of by-passing the governor in case something goes wrong with it.


On a multiple unit train and when locomotives are coupled to operate in multiple, the compressor operation is usually synchronised. This means that if one compressor governor detects low air pressure, all compressors will switch on together throughout the train. When the last governor detects the air pressure is restored to its proper level, all compressors switch off together.


Each compressor set-up will have its own storage reservoir, normally called the main reservoir. This is a pressure-tested vessel, capable of storing enough air for multiple operations of all the equipment on the locomotive plus the train brakes. If there is more then one compressor, there will be more main reservoirs. Most modern locomotives have several and a multiple unit train will often have one on each car, whether there is a compressor on the car or not. Individual items of pneumatic equipment will also have their own storage reservoirs. It is not a good idea to run out of air, particularly for brakes! In New York City, this was carried to extremes, where some trains had a compressor on every car of an 11-car train.


Once compressed, the air has to be distributed around the locomotive and along the train. Normally, for a freight train, the air is only needed for control of the braking system and a "brake pipe" is run the length of the train to achieve this. The details are in the Railway Technical Web Pages North American Freight Train Brakes Page. For locomotive hauled passenger trains too, a brake pipe is normally sufficient but for multiple unit trains, a compressed air supply is usually provided on every car.

EMU Compressor Setup - comp-03.gif (12937 bytes) Compressed air distribution along a multiple unit train (see the drawing left) is by way of a "main reservoir pipe" (MR pipe), sometimes called a "main line pipe". The pipe is usually connected between cars by hoses. Each vehicle carries half the hose and is connected to the next car's hose by a cast steel coupling head which is designed to fit its opposite number. The heads will automatically disengage if they are forced apart by the sudden uncoupling of the train. They do this because, when the hoses become horizontal as the cars part, the heads reach a position where they uncouple.

Most of the standard equipment is listed in the drawing above but most EMU trains use air pressure to raise the pantograph (if fitted) and some third rail trains use air pressure for control of shoe contact with the current rail.

Angle Cocks

Most EMU vehicles have a MR pipe "angle cock" at each end. The angle cock can be closed to shut off the air supply at that point. Before uncoupling a vehicle, it is normal to close the angle cock on either side of the uncoupling position. This prevents any kick from the pipe as it is disengaged. Closing the angle cocks also has the effect of bleeding off the air trapped in the hose. The angle cock has a special bleed hole for this purpose.

Automatic Couplers

Many EMU's are provided with automatic couplers, usually at the ends of the unit. The coupler provides for all electrical, mechanical and pneumatic connections and is usually remotely operated from the driver's cab, or at least, inside the car. In the case of the MR pipe connection, a valve will open to provide the connection to the next unit once the cars are confirmed as coupled.

Sometimes, automatic couplers are operated by a compressed air supply. This is used to provide power to engage and disengage the mechanical coupling and to open and close the connecting valves and contacts.

Air Operated Equipment

Apart from automatic couplers and brakes, already mentioned above, there are a number of items on a train which can use compressed air for operation, although the modern trend is away from air in favour of electric systems. There are some simple items like the horn and the windscreen wiper and some more complex ones like traction control and door operation. Each item will have its own isolating cock to allow for maintenance and most of the larger systems have their own storage reservoir.

Many systems do not need the full main reservoir air pressure of 6 to 7 bar (120 to 140 lbs./inĀ²), so they are equipped with reducing valves on the upstream side of the reservoir. Some are equipped with gauges as well, although most engineers prefer just a test plug. Gauges stick out and get knocked off too easily. Nothing drains a reservoir more quickly than a broken gauge.

Traction Equipment

Although electrical operation of traction control equipment is the most common, some traction control systems use compressed air to operate circuit breakers, contractors or camshafts. There is normally a traction control reservoir and its associated isolating cock provided for each vehicle set of equipment.


Many rapid transit and suburban trains still use air operated door systems, controlled from the cab at one end of the train but using air stored in reservoirs on each car. The reservoirs are replenished automatically by way of their connection to the main reservoir pipe. Door systems usually use lower than normal MR air pressure. However, electric operators are the preferred option these days.

Air Suspension

Placing the car body on air pressure springs instead of the traditional steel springs has become common over the last 20 years for passenger vehicles.  The air spring gives a better ride and the pressure can be adjusted automatically to compensate for additions or reductions in passenger loads.  The changes in air pressure are used to give the brake and acceleration equipment the data needed to allow a constant rate according to the load on the vehicle.

Driver's Brake Control

Most trains use compressed air for brake operation.  Most locomotives and older EMU's use a pneumatic brake control system which requires a brake valve to be operated by the driver.  The valve controls the flow of air into and out of the brake pipe which, in turn, controls the brakes on each vehicle in the train consist.  The driver's brake valve is connected to the MR pipe in the cab so that there is always a constant supply of air available to replenish the brake control system when required. An isolating cock is provided in the cab so that the brake control can be closed off when the cab is not in use.

Pantograph Compressor

One additional compressor is often provided on units which have air operated pantographs, i.e. those which are raised or lowered using compressed air as the power medium.  Opening up a completely dead locomotive is only possible if there is battery power and some compressed air available to get the pantograph up to the overhead power supply.  After all, nothing will work on the loco without power.  So, a small, battery powered compressor is provided to give sufficient compressed air to raise the pantograph. As soon as the pan is up, full power is available to operate the main compressor.

See also the Rolling Stock Pages Links Page - Multiple Unit Operation - Electronic Power - Electric Traction Pages Drives -  Electric Traction Pages (DC Resistance Control) - Electrical Auxiliaries - Electric Traction Pages Glossary pages.