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Our collection of UK railway, general control, electrical and DCC terms with some computer jargon thrown in!

This list is almost certainly not exhaustive, and is currently (and probably for ever) a work in progress, but we hope it will assist!

We offer our thanks to the many sources, books, websites, museums and railway workers who have contributed to this, and are too numerous to name. This is a work in progress! Please Contact DCC Supplies email us if you wish to add terms, or make corrections to the inevitable errors!


4 foot
A commonly used term for "between the rails" i.e. the track (UK standard gauge being 4 foot 8½ inches).

6 foot
Commonly used term for the space between two adjacent tracks.


Absolute-Block Working
A system of train operation in which only one train at a time is allowed inside a block section. (see also permissive-block working).

Lenz system of auxiliary control for DCC decoders.

An alternating current (AC) is an electrical current whose magnitude and direction vary cyclically, as opposed to direct current, whose direction remains constant. The usual waveform of AC is a sine wave, as this results in the most efficient transmission of energy. However in certain applications different waveforms are used, such as triangular or square waves, with DCC the track current is an AC sine wave, with the commands superimposed. on the base signal.

Active device
A device or circuit that requires an electrical power supply to operate. An example is a locomotive decoder.

Acceleration and deceleration delay
Setting on a locomotive decoder. The delay determines how fast the loco reaches it set speed.

Accessory decoder
A decoder that is not intended to be installed in a locomotive, but remains in a fixed location and controls accessories such as signals or track switches/turnouts.

The numeric identification code by which a decoder recognises commands directed specifically to it. It is also the identifier that a transponder broadcasts. The address is usually unique for each decoder, but this is not a requirement, this can be useful when more than one decoder is installed in a locomotive or as part of a multiple unit - i.e. a HST set, the motor and forward lighting controls in the Driving unit, and a two channel decoder for lighting in the rear DVT.

Advance direction (or in advance)
Forward from the location, in the normal direction of travel, for the line in question.

Advanced-starting signal
Where it is necessary to have two or more consecutive starting signals within station limits for any particular line, the furthest advanced of them is the "advanced starter".

Air bags
Slang term for the flexible pipes used to connect air brake systems between vehicles. (see BAG)

Airfix MTC
In the 70's Hornby and Airfix both produced digital systems, the Airfix version had four separate controls, for separate trains.

The unit of measure for electrical flow, can be thought of loosely as the electrical equivalent to litres per second.

Analogue Control
The 'Traditional' method of control. Places a DC voltage on the track, and controls the voltage by means of a rheostat. Direction is controlled by reversing the DC supply. Locomotives are stopped by removing the voltage from the track. This system in its simplest form can only control one locomotive at a time. By isolating locomotives in sidings or by installing cab control, more than one locomotive can be operated.

An element of colour light signalling. For instance, 2 aspect is simple Red/Green, 3 aspect is Red/Green/Amber, and 4 aspect if Double Amber, Amber, Red & Green

Asymmetrical DCC
Asymmetrical DCC can be used to enforce speed control through important sections of your layout or to stop a train in a siding, in the station, or before an occupied rail crossing. Using a few diodes, you can even protect a train from overrunning a turnout thrown in the wrong direction preventing unintended shorts, which could shut down a section of the railroad.

Automatic block
A block section in which operation of the signals is fully controlled by track circuits.

Automatic block signal
A signal controlling the entrance to an automatic block.

Auxiliary signal
Early name for a distant signal.

Avoiding line
A railway route which diverges from a main route and rejoins it again at another location after avoiding an area of congestion.


Back Light
The white light that is visible from the back of a signal lamp, used to inform the signalman visually that the lamp is lit.

Backing Signal
A subsidiary signal which is used to control shunting movements within station limits in a direction which is wrong for the line in question.

Old railway term for a flexible pipe, usually used for brake pipes between rolling stock, or the water feed from a tower.

Hard core used to provide a stable & solid base with good drainage for the track. Usually granite Chippings between 1-2 inches diameter are used, although cinders and other material is not uncommon.

A banking locomotive was often used to assist trains up particularly steep or long banks, or gradients. Many gradients had locomotives permanently allocated for this purpose. Trains would stop at the bottom of a bank, and the banker would buffer up to the rear (but not couple on). The train would then ascend the bank with the bankers assistance, and at the top the banker could drop off without the train needing to stop. N.B. Only DCC can provide this in the modelling world!

Bell Code
The sequence of bells (block bells) used to communicate between signal boxes using a standard code.

Block Section
The length of track between the most advanced starting signal controlled by one signal-box and the outermost home signal controlled by the next signal-box. Entrance to this section of track is controlled by a signal which for safety reasons is normally at danger and hence the section remains "blocked" by this signal until a "line-clear" telegraphic-message is obtained from the block post in advance. With automatic signals, a block section is controlled automatically by train detection or remotely by centralised train control.

Used to blow steam out of the chimney creating a through draft to draw the fire through the boiler tubes. Used when the regulator is shut and when there is no exhaust steam to create the draft. Without a draft, blow-backs into the cab are likely whenever the firebox doors are opened. This situation most likely would occur when entering a tunnel.

British Rail
Formed in 1948 by nationalising the existing 'Big Four' railway companies, Great Western Railway, London Midland and Scottish, London North Eastern Railway and Southern Railway. British Rail operated the entire national railway network until privatisation in 1995, since when various sections have been sold to other operators.

Buffer Lamp
A red lamp set on buffer stops.

Buffer Locking
The state of affairs where a curve is so sharp or the change of radius is so sudden that one buffer of each of two adjacent vehicles are displaced so much that they no longer touch; and, when the vehicles straighten up, the buffer heads of one vehicle slip behind those of the next vehicle.

Some locomotive decoders can sense the rotational speed of the motor and automatically adjust future digital pulses to the motor to maintain a desired speed. Also called load-compensating decoders. Back emf is the voltage developed by the spinning motor armature as it acts as a generator. EMF is short for electro-motive force. Back emf measurements are used for speed stabilisation on areas of the track such as hills, or long curves where additional motive power is needed to compensate for gravity or friction.

Make Superb rolling roads! Very useful when programming DCC decoders

Braking sections
Track segments where the power supply is set up so that DCC trains stop automatically. An example is ABC or Asymmetric DCC

A set of wires or tracks that serve as a conduit for electrical signals and distributes them around the layout. Usually a heavy duty pair of cables, to ensure low resistance and a stable voltage around the layout. The bus should be laid with the wires as parallel as possible to each other. Resist the temptation to separate them or take different routes around obstacles and other cables, as this can cause RFI and also interfere with other model rail systems. Our copper tape (in 5, 12 and 25mm width x 33metres) is ideal for this, permitting easy and neat running of a bus round your layout.


The driver location, can be single ended or double ended. Not always located in the same vehicle as the power unit! (see DVT)

Cab Control


Capacitor discharge

Command Station
The command station receives electrical signals (operator instructions) from the cab. The command station then creates NMRA DCC digital packets in accordance with NMRA specifications to achieve the desired results and transmits these packets to the power station.

Configuration variable (CV)
Memory location in the decoder that contains information that controls the decoder’s characteristics. A defined piece of information used by the decoder to adjust its operation. This information is permanently stored inside the decoder until the user wishes to change its value.

Operating and controlling several locomotives as if they were a single entity. There are three types of consisting: (1) Basic consisting is where all locomotive decoders in the lash-up have the same address. (2) Advanced consisting is where the consist information is stored in CV19 in the decoder. (3) Universal consisting is where the consist information is stored in the command station.

Calling-on Signal
Used to indicate that a driver may move passed a stop signal at danger to a predetermined location, or the adjacent signalbox from where he would receive further instructions.

Catch Handle
The small spring-loaded locking-lever or `catch' at the end of a points or signal operating lever. The catch must be raised before it is possible to move the lever proper.

Catch-Handle Locking
A system of interlocking in a lever frame, where the mechanism is activated by raising the catch handle rather than moving the lever proper.

Catch Points
Catch points are used to deliberately derail vehicles running away in the wrong direction on a running line, or to protect the running line from a siding which is on a gradient, in this case, the catch is placed so as to direct the runaway, away from the running line, so as to prevent fouling of the line. See also: Trap Points.

That part of the overhead wiring on electric railways, which hangs between the supporting masts and from which the dropper wires are hung.

The area alongside and below the level of a railway track into which rain water is drained.

A guiding rail, located between the two running rails, and set close enough to one of the running rails to make contact with the back of a flange. Hence, such contact will prevent the opposite flange from making hard contact with the running rail on a sharp curve; or prevent the opposite flange from taking the wrong route at a rail crossing.

Clearing Point
A position 440 yards in advance of the outermost home signal of station limits, and to which position the line must be clear before a train can be accepted into the block section in rear.

Co-Acting Signals
Signals in which the indication is duplicated in two different positions on the same post.

Colour-Light Signal
Signals which are indicated by colour lights only, and which are bright enough to be observed in daylight. Colour light signals are described as multiple - aspect.

Contact Wire
That part of the overhead wiring in railway electrification to which the pantograph of the moving vehicle makes contact.

continuous current
is the constant flow of electric charge from high to low potential. This is typically in a conductor such as a wire. In direct current, the current flow is in the same direction, distinguishing it from alternating current (AC).

A department of a railway organisation which makes decisions and directs the movement of trains and train workers.

Cross-Country Line
A railway route which inter-connects two or more main trunk routes. Some routes which are cross-country lines according to this definition, but are of minor importance, might be known as branch lines.

One rail crossing another on the same level.

Crossing Angle
The acute angle that one rail makes relative to the other at a crossing.

Two inter-connecting sets of points which allow a vehicle to cross from one track to a parallel track.

Usually quoted as a percentage, this is the point in a piston stroke at which the supply of steam to the cylinder is cut off by the closing of the valves. Maximum cut-off (typically 75%) is used for starting a train and is reduced as the train accelerates and less effort is required. The amount of cut-off used is controlled by the driver via the reversing lever, or screw reverser in later designs. This process is also referred to as "notching up", this process produces a distinctive change of sound in the 'Chuff' or 'Bark' as if it has been 'cut-off'

A hollow cylindrical-shaped device in which the power of a reciprocating engine is developed. In the case of a steam engine, it is here that the power of the steam is converted to mechanical motion.

Cylinder Drain Cocks
When a steam locomotive has been standing for a while, steam may condense to water in the cylinders as they cool. The water, being incompressible, can result in damage to the cylinders and pistons when the locomotive then tries to move away. To prevent this, cylinder drain cocks are fitted; controlled from the cab, they should be opened while the locomotive is standing and not closed until after the locomotive starts to move. It is these that produce the typical large cloud of steam and sound! from the front of a locomotive when it first begins to move.


Direct Current - there is a measurable positive (+ve) and negative (-ve) charge, this is the 'traditional' way of powering model locomotives, either by 2 or 3 rail systems, or by overhead cantenary. DCC systems are based on an AC supply.

Digital Command Control. The control information is provided in the form of a digital signal instead of a standard analog (DC or AC) power, overlaid with control information. NMRA DCC is a specific form of Digital Command Control specified by the NMRA as a non-proprietary international specification and is implemented by a significant number of manufacturers worldwide. On the most basic level, DCC encompasses systems and products that are interoperable with the basic NMRA DCC Standards.

Diamond Crossing
The crossing of one train track with another on the same level.

Diesel Electric
Diesel Electric locomotives are effectively electric locomotives which carry their own generating plant. The diesel engine and generator typically being directly coupled together, and the motors being mounted on the bogie frames.

Diesel Hydraulic
Diesel Hydraulics are largely mechanical in principle and rely on the properties of liquids, namely that they are practically incompressible, as such they can be used to transmit force that can be guided into well defined channels. A hydraulic torque converter is basically a pump and a hydraulic turbine in a common casing. Since there is no direct mechanical connection between input and output, an infinitely variable range of speeds is possible between fixed limits. Power is transmitted to the wheels via mechanical drive-shafts from the torque converter/gearbox.

Diesel Mechanical
Diesel locomotives with mechanical transmission rely upon gear boxes in a similar way to most cars and drivers therefore have to change gears regularly as the speed varies. Usually only used for low power shunting locomotives.

Disc Signal
A semaphore type signal where the indication is given by a rotating disc.

Directional Lighting
When a locomotives running lights operate prototypically in the direction of travel. i.e. white at the front, and red at the rear, automatically changing over when direction of travel is reversed. This is achieved by the use of a diode, or by a DCC decoder.

The disposing of the remains of the fire at the end of a days working for a steam locomotive. This consists of three main tasks, removing the remnants of the fire from the grate, emptying the ash-pan and emptying the ash out of the smoke-box.

Dead Frog See Insul-frog

See: Locomotive Decoder or Accessory decoder

Diamond crossing

Digital command control

Distant Signal
A caution signal which when in the on position, indicates that the next stop signal is at danger, or at least one of the stop signals within the approaching station limits is at danger.

Is digital pulse power, it provides better and smoother low-speed performance and is especially helpful on locomotives that have high starting friction. Try experimenting with Dither to achieve better low-speed operation, the effect will narrow the window between what point the locomotive starts to move and at what point it will stay running.

A diode is a component that restricts the direction of an electrical flow It allows a current flow in one direction, but essentially stops it in the opposite direction. The diode can be thought of as an electronic version of a one way valve. Circuits that require current flowing in only one direction will typically consist of one or more diodes in the circuit design. This device is useful for the railway modeller when used with directional lighting, or to produce Asymmetric DCC also useful for preventing locomotives from hitting buffer stops in sidings or termini, but permits exit when the current is reversed (only applicable to DC analogue control)

Common acronym for Diesel Multiple Unit.

Double Header
A train which is hauled by two locomotives coupled together.

Double Slip
The combination of a diamond crossing of small crossing angle interlaced with four sets of points so as to provide a route between any two opposite tracks. When four of the point blades are located totally within the central diamond, it is called an "inside slip", and when all point blades are outside the diamond, it is called an "outside slip".

Double Track
A railway route in which one track is provided for each direction of travel.

Down Line
The track on a 2 or more track main line where the direction of travel is away from the major city, (usually London.) Where there are more than one directional line, it is know secondarily by function - i.e. 'Down-main', 'Down-relief', 'Up-Fast', 'Up-Slow' Etc.

Double pole, double throw

Double Pole, Single throw

Draw-Ahead Signal
A subsidiary signal, usually mounted on the same post as a starting signal. When the draw ahead signal is off, the train may draw ahead cautiously against the starting-signal's indication, but only as far ahead as the line is clear, or as far as an advanced starting signal (see also calling-on signal, shunt ahead signal and warning signal).

Drop Head Buckeye Coupling
A buckeye coupling that can be lowered to reveal a standard hook for a screw or three link type coupling, examples can be found on some class 66's.

Dropper Wire
Wires hanging from a catenary on to which the contact wire for railway electrification is attached.

Dynamometer Car
A railway vehicle which incorporates apparatus for measuring draw-bar pull, speed, and work done by the locomotive to which the vehicle is coupled.

Driving Van Trailor


Most steam engines used for passenger and mainline work generally, had vacuum brakes. The vacuum was created by blowing steam through a series of cones. The larger cone (large ejector) was used to create the vacuum before the locomotive/train could move. A smaller ejector could be used to maintain the vacuum while running, until (if fitted) the vacuum pump had enough speed to provide the required suction.

A type of locomotive that can work as an electric locomotive where electrical supplies exist, but also has a diesel generator for use elsewhere. These locomotives are designed for electric operation for most of their work and utilise the diesel generator only in yards where there may be no supply or for short journeys. The diesel generator usually only produces a fraction of the power available where electric supplies are available.

Electromotive force

Common acronym for Electric Multiple Unit.

Escape Road
A loop adjacent to a platform track at a terminus station, which enables a locomotive to "escape" from the dead end of the terminus, in which it would otherwise be trapped by its coaches.


Expansion Joint
A special rail joint in which each rail is cut back and overlapped in such a way that considerable longitudinal movement between the two rails is possible, preventing buckling due to variations in temperature. Commonly used in continuous welded rail (CWR) sections.


Function-only decoders
installed in equipment that moves, but function only decoders do not control movement. Rather, they control other functions like lights, sound, smoke or animation.




High Speed Train


Infra-red (I.R.)








Live Frog

See Memory Location or Train Location Device. Also simply where a train or item if rolling stock is.

Locomotive Decoder
Electronic device that receives the DCC signal from the command station through the track, decodes it and tells the locomotive it is controlling, what to do.



Memory Location
See Configuration variable





Normal (Points)

Normally Open (N/O)
A term for a switch or circuit that is normally not conductive to electricity (i.e. normally OFF) When activated, the circuit or switch will close (switch ON)

Normally Closed (N/C)
A term for a switch or circuit that is normally conductive to electricity (i.e. normally ON). When activated, the circuit or switch will open (switch off)

National Model Railroad Association

The US based association that has become the international test and certification body for DCC equipment. The NMRA was initially setup as a national resource for American railroad modellers to share information.


Occupancy detector
A device which senses and provides feedback for the presence of a train or specially equipped cars on a section of track. it maybe a passive or active device.

The unit of electrical resistance.


Constant 'end stops' which limit a range of information, or a single item of information, which are used to provide a range of control. i.e. in a range of 0-10, which increments in steps of 1, the numbers 0, 1 and 10 are the parameters.

Passive device
This is a device that reacts to external stimulus without requiring its own power supply. An example would be a reed switch.


Potential Difference
Potential can be thought of as of as "electric pressure". Where this "pressure" is equal nothing happens, just as we do not feel atmospheric pressure. However, where a difference in pressure exists between two points, it produces a force that can push charged objects to different locations.

Power bus
Main wires that carry the power from the booster to provide power feeds to the power district.

Power District

Power station
Booster is the electronic device that combines and amplifies the DCC commands generated by the command station with power from the power supply. The booster sends the DCC commands as electronic signals along with the track power to the decoders to deliver both power and DCC signals to the DCC devices on the layout. A DCC system may have more than one booster. Boosters are also sometimes called power boosters or power stations.

Power Supply
A source of an electrical voltage, this could be AC or DC, and maybe either Stabilised, or Un-Stabilised.

Programming on Mainline (PoM)
(Also known as programming on the main.) Method where programming information is sent to a specific decoder on the layout instead of on the programming track. This method of programming decoders does not interfere with the operation or settings of other decoders on the same track. A programming track is not used, the information sent is directed to a specific address. Not all decoders accept PoM.

The action of setting the internal parameters of decoders and other control equipment. During programming, values are set for CVs to determine the personality of locomotives, accessory decoders and other programmable DCC devices.

Programming track
An isolated track section used for programming decoder equipped locomotives or transponder equipped rolling stock.



Reed Switch
This is a passive device, which when a magnet is placed nearby closes (or opens) an electrical circuit. They are available in normally open (N/C), or normally closed (N/C)

Resistance (Electrical)
The natural difficulty that an electrical current has when passing through a conductor. Measured in Ohms. Resistance in a circuit produced heat, and reduces the voltage, hence the need for a power bus.

Revesed (Points)

Reversing loop





Rolling Road


Scissor crossing

Single pole, Double throw.

Speed Steps
Controllable voltage increments which are used to control motor speed. With some decoders, the output power can be set for each speed step. The range from zero to full power is divided equally into 14, 28 or 128 speed steps.

Single pole, Single throw.

A term applied to a power supply that provided its rated output of voltage and current output regardless of the load.

Stall Current
The maximum current draw in amps that a locomotive is capable of when stalled. When a motor is prevented from rotating and its maximum rated voltage is applied, the current draw of the motor is known as its stall current. Typically, it is safest to insure that the stall current rating of a locomotive decoder exceeds the stall current of the motor being controlled. In case of a derailment or gear bind and subsequent motor stoppage, the decoder will not be damaged.




Throttle Notches
Determines whether a loco is controlled with 14, 27, 28 or 128 speed step mode.


Train location device


USP Technology - Uninterruptible Signal Processing
Most decoders need a clean and constant connection to the track to pick up the DCC signal. Even a small section of dirt can prevent the commands from getting to the locomotive. Not so with USP Technology! With USP, the decoder can read the signal on either rail or even if the locomotive is entirely on dirty track.


The contents of a CV or Memory location.

Voltage is an electric potential difference between two points, the unit of measure is the volt.




The Lenz data bus standard; High speed data communication network for connecting Digital plus input devices together.

(XBUS) input devices
Devices that are used on an XpressNet bus to control your layout. i.e. handhelds, cab tower, interface, etc.


'Y' Point


Zero One

Austrian manufacturer of extremely high quality and technically advanced DCC decoders. Though there are many advanced features, ZIMO equipment is simple to use, and remarkably intuitive and comfortable to operate.

UK DCC manufacturer, has a cab like layout to it's controllers.