Train Equipment


Here we discuss the auxiliary services required by trains and how they are arranged on locomotives and passenger vehicles. Put simply, a train needs a power source, a drive system between the power source and the wheels, a brake system, a control system, hotel power for such things as lighting, battery charging and HVAC plus a compressed air supply.

Power Sources

There are two sources of power for trains - on board or external. The on board system requires the train to carry its own fuel - coal in the case of a steam locomotive or fuel oil for a diesel locomotive. The arrangements for a typical diesel locomotive are shown in Figure 1. The external power source system is electric, where the train collects current from a sliding contact with a power supply line. The power supply line can be a third rail or overhead wire, see Electric traction Power. A typical arrangement for an electric locomotive is shown in block diagram form in Figure 2.

Figure 1: Block diagram of a diesel electric locomotive showing the basic arrangement of the equipment. The locomotive has to carry its own fuel to provide energy for the diesel engine. The engine drives a main alternator to provide power for the electric motors and an auxiliary alternator which provides hotel power for the locomotive and the train it hauls. Diagram: Author. 

Figure 2: Block diagram of an electric locomotive showing the basic arrangement of the equipment. The locomotive collects electrical energy from an overhead line or third rail. The current passes through a transformer and converter to provide power for the electric motors via a power control unit. Power is also used to provide hotel power for the locomotive and the train it hauls. Diagram: Author. 

On-Board Services

The modern passenger train has 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.  Many different types exist. Their use and features can be summarised as follows:

Compressed air - Train safety requires that an effective brake system is available at all times. One of the oldest and most reliable braking systems used on trains employs compressed air. Other systems on trains have used compressed air, such as door operation, traction systems, suspension and coupler operation.

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 and now the preferred solution.  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.

Electrical Systems

A locomotive or multiple unit is provided with two electrical systems, high voltage (HV) and low voltage (LV).  The high voltage system provides power for traction and a source of power for the low voltage system.  The low voltage system supplies all the auxiliary systems on the train like lighting, air conditioning, battery charging and control circuits.  The two are separated because the high voltage required for traction is not needed for most of the other systems on the train so it is wasteful and expensive to use the high voltage.

Converting HV to LV

The current drawn by a locomotive from the overhead line or third rail supply can be supplied at voltages ranging from 25,000 volts AC to 600 volts DC.  With the exception of heaters and compressor motors which, on the lower voltage DC railways are normally powered by the line current, all of these supply voltages are really too high to use efficiently with the comparatively small loads required by the on-board services on a train.  The common approach therefore, has been to reduce the line voltage to a suitable level - generally below 450 volts and on some systems as low as 37.5 volts.  Most systems have used a dynamo, a generator, an alternator or a current converter to get the lower voltages required.  Usually, different voltages are used for different applications, the particular conversion system being specially designed to suit.


The first electric lighting provided on steam hauled trains was supplied from a large capacity battery contained in a box slung under the coach.  The battery was recharged by a dynamo powered by a belt driven off one of the coach axles.  Of course, this meant that the battery was recharged only when the train was moving and it had to have sufficient capacity for prolonged station stops, particularly at terminals.  The voltage varied from system to system but was usually in the 12 to 48 volt range.  Trains were heated by steam piped from the engine.  If the locomotive was electric or diesel, a train heating boiler would be installed on the locomotive.  Some European railways had train equipped with both steam and electric heating.  More recently, all heating has become electric.

Electric trains originally used power obtained directly from the line for lighting and heating.  The lamp voltage was kept to a low level by wiring groups of lamps in series.  Each vehicle had its own switch which had to operated by a member of the crew.  On some railways, where there were tunnels, daytime crews were instructed to switch on all the lights at the station before the tunnel and switch them off at the station after the exit.  Such stations were provided with staff allocated to this job.

Motor Generators

In the mid 1930s, electric multiple units began appearing with on-board, DC generators to supply lights.  This allowed lower voltages and reduced the heavy wiring required for traction current fed lighting.  Outputs from these generators ranged from 37 to 70 volts, depending on the application.  The generator was driven by a small electric motor powered by the traction supply.  For this reason they were often referred to as "motor generators".

The motor generator system, the train lighting and battery are fed from a generator driven by a motor at the line voltage.  The return circuit is through ground, using the car structure like a road vehicle.  A voltage regulator is provided to reduce the risk of damage through sudden changes in voltage caused by gaps in the current rail or neutral sections in the overhead line.  If the MG stops, the battery is disconnected from the charging circuit and supplies a few emergency lights.  In addition to supplying lighting, the LV circuit was used to supply all the train's control circuits.  See Multiple Unit Operation. 

Motor Alternators

By the late 1940s, fluorescent lighting was becoming popular and was recognised as better, brighter and requiring less current that tungsten bulbs.  However, if DC is used, the lighting tubes become blackened at one end, so AC was adopted for lighting circuits on trains.  At first, some systems used a DC generator with an alternator added to the drive shaft, a motor-generator-alternator.  The DC output from the generator was used for control circuits while the AC output from the alternator was used for lighting.  Emergency lighting was still tungsten, fed from the battery. 

Figure 3: Schematic for a DC electric supply system on an EMU. Current collected by the pantograph (or shoe on a 3rd rail system) is divided between the various systems on the train. Low voltage supplies for lights and control systems are provided by a motor alternator/rectifier/battery system. Diagram: Author.

In the early 1960s, the motor alternator appeared.  The appearance of silicon rectifiers allowed the AC output of the alternator to be converted to DC for battery charging and control circuits.  The introduction of solid state electronics also saw the old mechanical voltage regulators replaced.

Electronic Auxiliaries

Modern auxiliary services on electrified railways are now mostly solid state systems, using power and control electronics, as shown in Figures 4 and 5.

Figure 4: Schematic of a 25kV AC overhead system but it is similar for DC systems except for the absence of the transformer and AC-DC converter. The output from the DC to AC auxiliary converter is 3-phase AC at about 380 volts and is used for train lighting and the AC motors of air conditioning fans and compressors.  The 3-phase is also converted to DC by the rectifier which provides current for battery charging and control circuits.  

Figure 5: Schematic block diagram of the main and auxiliary electric systems on a diesel electric locomotive. Diagram: Author.

On a locomotive hauled train, the individual coaches are provided with an on-board converter supplied from a train line carrying a 3-phase supply generated on the locomotive.  On a diesel locomotive, this supply would come from an on-board alternator driven by the diesel engine.


A feature of electric railway operation is the gap or neutral section.  Gaps occur in third rail systems and neutral sections in overhead line systems.  See also Electric Traction Pages Power Supplies.  The gap in a current rail is necessary at junctions to allow the continuity of the wheel/rail contact and at substations to allow the line to be divided into separate sections for current feeding purposes.  Neutral sections in the overhead line are also used for this purpose.

Although they are always kept as short as possible, gaps will sometimes cause loss of current to the train.  The train will usually coast over the gap but there will be a momentary loss of current to the on-board equipment - lights will go off for a second or two and ventilation fans will slow down or stop.  On trains provided with generators or alternators, the momentum in the machine would often be sufficient to maintain some generation over the gap and lighting often remained unaffected.  The only difference noticeable to the passenger was the change in the sound of the generator as it lost power and then regained it a second or two later.

Modern electronics has given us static inverters to supply on-board inverters but they have no inertia and stop output as soon as a gap is encountered.  To prevent the lights going off at every little gap, all lights are connected through the battery.  To prevent the battery becoming discharged too quickly, the inverter starts a "load shed" at about a 60 second delay.  After this time, the main lighting is switched out and only emergency lights remain.  Battery current is also used for emergency ventilation, essential controls and communications.

See also the Multiple Unit Operation, Electronic Power, Electric Traction Drives, Electric Traction Pages (DC Resistance Control), Pneumatic Auxiliary Equipment and Electric Traction Pages Glossary pages.

Air Equipment

Looking at compressed air supplies, Figure N 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.

Figure 6: A schematic of an air supply system on a multiple unit train. Not every train has all the air operated equipment shown here. Diagram: Author.

The Compressor

Compressed air is almost always used for brakes and sometimes for powering train doors. Also once popular for powering traction power switches or contactors. It is usually used for raising pantographs on overhead line systems. Compressed air 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). 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.

Figure 7: A typical EMU air compressor, designed to be fitted under a train floor. Photo: Atlas Copco.

Compressor Drives

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 than 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.

Compressed air distribution along a multiple unit train (Figure 6) 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 shown in Figure 6 above but many EMU trains use a separate battery operated compressor 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./in²), 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.

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