- Drain Cock
- When steam enters a cold cylinder it condenses into water. Water is incompressible, and so Drain Cocks are left open when an engine starts off, and closed once the engine is on the move. It is steam escaping from the Drain Cocks that gives the very distinctive plume of steam at the front of the engine (just above track level) when a steam train departs. Drain cocks are operated from the footplate
 . Also known as Cylinder cocks.
- Draughting
- The process of adjusting the flow of air through the firebox, tubes and out through chimney to make the most efficient use of the coal.
- Draw Gear
- The coupling(s) and associated springing mechanism on a locomotive, carriage or wagon.
- Draw-bar horse power
- The power available for pulling the train; i.e. indicated horse power less the power required to move the engine (and tender).
- Dray
- Horse-drawn road vehicle for goods collection/delivery to and from traders’ premises and the railway’s goods depot. Post-WW1 motor vehicles began to replace the horse-drawn drays, but these were still in use until c.1950. In later years horses were replaced by ‘mechanical horses’, that is motorised tractor units.
- Drayman
- Also often called a “carter” or “carman”, a person who drove a horse and cart (or “lorry”, “lurrie”) engaged on goods collection and delivery from/to a railway goods yard or depot. A drayman would have the same horse permanently assigned to him, and would often groom the animal in his own time. Just as today, in former times there were continual complaints about traffic chaos in large towns caused by innumerable goods vehicles parking in front of premises whilst loading and unloading. The amount of horse manure deposited in the streets was also a serious problem.
- Dreadnought Locomotive Class
- Designed by F.W. Webb, the ‘Dreadnought’ class was basically an ‘Experiment’ enlarged to produce more power. The boiler was larger, with 176 psi instead of 150 psi, and the valve gear was modified. In many ways the ’Dreadnought’ was the first modern Webb engine. In addition to a circular smoke box door, tommy-bar screw coupling, it was the first engine to have the new 3in thick tyres made of steel.
- Drive Axle
- The axle which is directly driven by the rod (“connecting rod”) from the cross head and piston.
- Drive Wheel
- The wheels which actually drive the vehicle along. In the case of a steam engine these will be the large wheels in the middle of the locomotive connected together by coupling rods.
- Driver
- The name given to the person who controls the operation of the locomotive. Called the “Engineer” in the USA end elsewhere. Drivers in the UK received no special training as mechanics beyond what they picked up as they went along, and hence were not always very knowledegeable about the workings of their engines.
- Drop-End Wagon
- A wagon whose ends could be lowered to facilitate the removal of a load — compare Drop-Side Wagon.
- Drop-Side Wagon
- A wagon whose sides could be lowered to facilitate the removal of a load; ballast wagons were commonly drop-side, so that the ballast could be shovelled out alongside the track where it was required.
- DSO
- District Superintendent’s Office
- Dual-Fitted
- A locomotive or vehicle equipped with both air and vacuum brake systems. Because the LNWR used vacuum brakes and the Caledonian Railway air brakes, WCJS vehicles were dual-fitted.
- Dumb Buffer
- A primitive type of buffer used on early wagons — a simple block, usually of wood, often an extension of the wagon underframe.
- Dundalk Newry & Greenore Railway
- Formed on 21st July 1873 when the Newry & Greenore Railway transferred its powers and assets to the Dundalk & Greenore Railway which on that date changed its name to the DN&GR. The LNWR always had a major financial interest in the line and in 1874 took over the borrowing powers of the DN&GR, effectively giving them control. Most of the line was closed on 31st December 1951.
- Duplicate List
- When a locomotive or carriage reached the end of its accountancy or depreciation life it was normally replaced in the capital stock by a new one, which in theory replaced the older machine by another of like kind (subject to technological improvement). The new locomotive or carriage took the running number of the one displaced so as to maintain the capital stock at a constant population.
However, in many cases the old loco or carriage was not physically worn out, and was capable of further use, in some cases for many years. It was therefore transferred to what was called the “duplicate” list (locomotives) or the “supplementary” list (carriages), and assigned a different number in the series reserved for duplicate stock. If the vehicle was later broken up, then the number would be re-used. The system was complicated, but one rule was that vacated numbers were not re-issued in the same half-year, so at any given time there were never two locomotives (or carriages) with the same number.
- ‘DX’ 0-6-0 Locomotive Class
- The ‘DX’ 0-6-0s were designed by John Ramsbottom, and incorporated all the usual Ramsbottom features; his own design of chimney top, safety valves and screw reverse, horizontal smoke box door, coupling rods with split ends and wedge adjustment, slotted splashers wooden buffer beams and brake blocks, no brakes on the engine and green livery lined in black.
- Dynamometer Car
- A small carriage containing test and measurement equipment determine the power output and speed of a locomotive. Used when measuring a new locomotive design to gauge its efficiency, or in comparative trials between locomotives.
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