Multiple Unit Operation


Originally derived from lift operation over a hundred years ago, multiple unit (MU) control has become the most common form of train control in use around the world today.  This page describes how it started and its development in the century to date.

See also Electric TractionElectric Traction DrivesElectric Traction Glossary and Electronic Power.


Origins - The Relay - The Contactor - Multiple Unit Control - Forward and Reverse


Electric locomotives were originally designed so that the motors were controlled directly by the driver.  The traction power circuits passed through a large controller mounted in the driving cab.  A handle was rotated by the driver as necessary to change the switches in the circuit to increase or reduce power as required.  This arrangement meant that the driver had to remain close to the motors if long and heavy, power-carrying cables were to be avoided. 

While this arrangement worked well enough, the desire to get rapid turnrounds on city streetcar railways led to the adoption of remote control.  The idea was that, if the motors could be remotely controlled, a set of driver's controls could be placed at each end of the train.  It would not be necessary to have a locomotive added at the rear of an arriving train to allow it to make the return journey.  A cab would be installed at each end of the train and the driver just had to change ends to change direction.  Once this idea was established, it was realised that the motors could be placed anywhere along the train, with as many or as few as required to provide the performance desired.  With this development, more but smaller motors were scattered along the train instead of building a few large motors in a locomotive.   This is how the concept of motor cars and trailer cars evolved.  Trailer cars are just passenger carrying vehicles but motor cars are passenger carrying vehicles which have motors and their associated control equipment. 

Multiple unit trains, as these trains became known, were equipped with control cables called train lines, which connected the driver's controls with the motor controls and power switches on each motor car.  The opening and closing of the power switches was achieved by electro-magnetic relays, using principles originally designed for lifts.  While the idea was being established on passenger trains, it was also adopted on locomotives.  It quickly became the standard method of control.

The Relay

The diagram below shows how a relay operates.

relay.gif (6601 bytes) A relay is really a remotely controlled switch.  In the diagram left, a power circuit contains a switch which is opened and closed by operation of a relay.   The relay is activated by a magnetic core which is energised when a controlling switch is closed.  As the core is energised, it lifts and closes a pair of contacts in a second circuit - usually a power circuit.  The current required for the relay is usually much lower than that used for the power circuit so it can be provided by a battery. 

In the diagram, the controlling switch is open, so the relay is de-energised and the power circuit contacts are open.  If the controlling switch is closed, as in the right hand diagram, the relay is therefore energised and its core magnet lifts to close the contacts in the power circuit.

Applied to a simple lift operating between two levels, a control switch on each landing could use relays to switch on the lift motor to move the lift up or down.  On a train, the controlling switch could be located anywhere on the train to activate the relays controlling the power to the motors.  The same principles can be used to carry out any other switching e.g. for lights or heating.   It represents a safe and simple way of transmitting commands to a number of equipments in a train and it is the foundation upon which multiple unit control was based.  On modern rolling stock, the relay is being replaced in many applications by electronic control, which speeds operation, eliminates the mechanical movements required and allows the miniaturisation of control systems.

The Contactor

As we have seen from the description above, the relay must have a current applied to it all the time it is required to be closed.  To avoid having current on to, say, a lighting control relay all the time, a different type of remotely controlled switch is used.  This is called a contactor.

contactr.gif (7956 bytes) The contactor is really a latched relay.  It can also be called (in the US) a "momentary switch".  It only requires current to be on for a "moment" for it to operate.  In order to keep the contacts closed once the control current is lost, the power circuit contacts are held in position by a mechanical latch.  When it is necessary to open the power circuit, the latch is released and the contacts drop open.

The contactor is operated by two coils, each with their own controlling switch.  In this case, the contactor is closed or "set" by pressing the ON button and opened, or "tripped" by pressing the OFF button.  Both ON and OFF buttons are sprung so that only a momentary current is used to activate the coil.

Contactors are widely used on trains and, for us, are a good example to demonstrate how multiple unit control works in practice.

Multiple Unit Control

The following diagram illustrates the principal of multiple unit (MU) control as applied to a 3-car train.

lightmu.gif (15503 bytes) In the diagram (left), the lighting on each car is switched on and off by a lighting contactor.  The contactor is latched closed when its "set" coil is energised or opened by the "trip" coil to unlatch it when required to switch off the lighting. 

All the lighting contactors on the train are connected to train wires, in this case one for "lights on" (in black) and one for "lights off" (in blue).  The ON and OFF buttons are in the driving cabs at each end of the train so, the lighting can be switched on or off from either end of the train. 

To prevent unauthorised use of the control buttons, most of the important circuits in the cab are isolated by a "control switch" or "cab on switch".  This is key operated and keys are only issued to qualified drivers or maintainers.  It also means that, in our example, lights can only be switched on or off from one end at a time.  The same principle, using contactors or relays, is applied to all other systems on the train - driving controls, braking control, heaters, doors, air conditioning, public address etc.

Of course, current for the equipment on each vehicle, as in this case, lighting, comes from a separate source - the auxiliary supplies - in the form of a battery, an alternator, an inverter or a power train line.

Forward and Reverse

How, one might ask, does one ensure that a number of locomotives or EMUs (or DMUs for that matter), coupled together to work in multiple, perhaps facing in different directions, will all respond to the driver's command to go in the same direction, say forward, from the cab where he is sitting?  How do you prevent one locomotive taking off in the wrong direction?  Well, it's built in to the wiring and it's simple, as shown in this diagram.

for-rev.gif (8373 bytes) Each power unit (whether it be a locomotive or EMU) has a forward wire and a reverse wire connected to a "Forward and Reverse" switch of one sort or another in the cab.  Looking at Unit 1, if the driver selects "Forward", the forward wire (in red) is energised and the "forward relay" (the arrow shows the direction of movement obtained for each relay) is energised to make the locomotive move in the forward direction. 

To ensure the correct direction is achieved by a second locomotive (Unit 2) that is coupled to the first, the forward and reverse wires are crossed over in the jumper cable.  If the second locomotive faces in the opposite direction to the first, its reverse wire (shown in black here) will be energised to make the loco run in the same direction as its partner.  To make sure this always happens, all multiple unit control jumpers have their forward and reverse wires crossed.

But, you might ask, what if the locos both face in the same direction?  You don't need the crossed wires in the jumper.  The crossed wires in the jumper will make the second loco go the opposite way.  No, that's been solved too.  So that the same jumper with the crossed wires can be used anywhere, the forward and reverse wires are also crossed ON each locomotive, only at one end, usually near the jumper socket.  Now, no matter which way round the locos are coupled to each other, and in what order, the forward command will always make all units drive in the same direction and the reverse command will make all units drive in the opposite direction.

One final point.  The jumper heads are designed so that they can only be inserted one way into the coupler socket on the locomotive, rather like a mouse plug on a computer.

Modern Control Systems

Modern systems use single wires or even fibre optic cables for controls. The system is sometimes referred to as "multiplixing", where a number of control signals are sent along a single wire. Some administrations require hard wired controls for safety systems like train braking but diverse programming can be used to make this redundant.