Glossary A


#4869 (In Topic #623)
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AC : Alternating current. Electric current which changes polarity in a regular cycle. It is the normal mains supply.
Accessory Decoder : Decoder that provides power and operational control of one or more layout accessory devices, such as turnouts, signals, cranes, animation devices, lighting, etc. Also known as a stationary decoder.
Address : Used by the command station to communicate with a specific decoder. It can be either 2- or 4-digit, depending on the system, and is typically part of the locomotive’s road number. Addresses are unique to each decoder.
Adhesion Factor : The ability of a locomotive to pull a heavy train without slipping.
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".
Advanced Uncoupling : A system of automatic uncoupling where the rolling stock are pushed over an actuating mechanism which uncouples the locomotive/wagons.
Air Brakes : Each vehicle of an air braked train will have a cylinder with pressurized air on both the `train pipe' and `reservoir' sides of the piston while the brakes are off. The brakes are applied by releasing the pressure in the train pipe side, the resulting pressure differential will put the brakes on. The pressure is originally supplied from the locomotive via the train pipe and a one-way valve ensures that the reservoirs on each vehicle are pressurized. The train pipe runs the length of the train thus providing a fail-safe system should the train part.
Airbrush : A miniature paint sprayer that gives a controlled application of thinned paint.
Alternating Current : An electric current that reverses its direction of flow at regular intervals. Each move from zero to maximum strength and back to zero is half a cycle. A full cycle includes excursions in both the positive and negative direction.
Ammeter : Meter used to measure current strength - for example, how many amps a locomotive draws when it stalls.
Ampere Or Amp : Unit used to determine the amount of an electrical current.
ARPS : Association of Railway Preservation Societies
Articulated Locomotive : A locomotive which is made up of several units, with each unit being pivoted to the next unit. Each unit is on a separate frame which is free to swivel relative to the frame of the next unit.
Ash Pan : Fitted below the firebox of a steam locomotive to catch the hot ashes falling from the grate. Typically a flattish tray with flaps known as dampers, which are used to control the admission of air to the fire from below. Later steam locomotive designs had hopper type ash pans which could be emptied much easier than the older type.
Auto Train : An early form of multiple unit where a steam locomotive could be controlled by the driver sitting in a carriage. Removed the need for the engine to run around the train at a terminus.
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.
Automatic Train Control : An early system invented by the Great Western Railway in 1910 to give drivers an audio warning of the status of the signal ahead.
Automatic Warning System (AWS) : A system provided to drivers to give an audio visual indication in the cab of the status of the signal ahead.
In the UK this system briefly works as follows; As the train approaches the signal it passes over a ramp between the rails, in which are placed two electromagnets (or a permanent magnet and an electromagnet). If the signal is green both magnets are detected by a receiver mounted between the axles. This causes a bell to ring and a warning disk shows a black aspect. If the signal is anything other than a green, only one magnet is detected which sounds a horn and a black and yellow aspect is shown. In this case the driver needs to accept the warning by pressing a button, otherwise the brakes will be automatically applied to stop the train.
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.
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