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DC : Direct current. Electric current of constant polarity as supplied by batteries or from AC supplies via a rectifier. This is the power source for most model railways today but may well change in the not too distant future.
DCC : Digital Command Control. The latest digital operating system for model railways and becoming increasingly popular.
DCC Bus : Heavy-gauge copper wire used to distribute the electrical power from the booster around the layout. Each booster has its own set of track bus wires. Accessory decoders may be connected to a track bus. The wire gauge of a track bus must be appropriate to the rating of the associated power station and load.
Dead Frog : Where the crossing vee of a model turnout (point) is moulded of plastic so as to keep the two track circuits through the turnout entirely separate.
Decal : A type of lettering material for models. The letters and numbers are printed on specially prepared paper and then coated with varnish. The lettering is applied by soaking the decal in water to dissolve the film between the ink and the paper and placing the layer of varnish and ink on the car.
Decoder : In DCC, small circuit board that receives digital packets of information addressed to it by the command station in accordance with NMRA standards. Mobile decoders are mounted inside locomotives and control the motor, lights, and sounds. Accessory decoders are used to control non-locomotive items such as turnout motors, signals, etc.
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.
Digital Command Control : Method of controlling multiple trains and accessories using digital communications packets to send commands.
Diode : A solid state device which allows electric current to 'flow' in one direction only.
Diorama : A small scene with great detail. Can be used to set off your models or to photograph a particular model. Can be scenes which are added to your model railway as they are completed.
Direct Current : Electrical current that flows in only one direction. Used for control of most model railroads before command control.
Disc Signal : A semaphore signal where the indication is given by a rotatable disc.
Distant Signal : A signal showing advanced warning of the next signal but has no stop indication itself.
DMU : Diesel Multiple Unit. A piece of rolling stock that is self powered and carries passengers or parcels. Each outward coach has a cab at the outer end which enables the driver to drive the unit in both directions without having to turn the unit. Very useful on smaller layouts and end to end.
Double Header : A train which is hauled by two locomotives coupled together.
Double Slip : In effect, this is four points and a crossover compressed into a very small space. Very useful if modelling a yard in limited space and where a head shunt is required.
Down Line : That line on a multi-track main line where the direction of travel is away from the major city.
DPDT Switch : A double-pole changeover switch. Used on reversing loops and triangular junctions.
Drop Light : The opening window in a carriage door, or other similar window which opens by dropping down into the coach body.
Dropper Wire : Feed wires soldered to the side or base of the rail and which then pass beneath the baseboard.
Dry Brushing : A technique where an almost dry brush is used to highlight details on a model with paint. Can be used for weathering models or for scenic work, rocks and stone walling, for example.
Dry Transfer : Transferable lettering or images on waxy clear film. The lettering material is made of thin plastic with pressure sensitive adhesive, and the letters or images are released from the waxed carrier sheet when rubbed onto a model.
Duckunder : A passage underneath layout benchwork requiring ducking or crawling to reach another part of the layout.


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