AAR

Association of American Railroads, the trade association that represents the common interests of the railroad industry in such areas as standards, public relations, and advertising.

AC

See Alternating Current.

ACC

See CA.

Access hatch

A hole in the benchwork and scenery, sometimes concealed with a lift out section of scenery, for emergency access to parts of the layout.

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.

ACI

Automatic Car Identification; see Kar Trak.

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.

Airbrush

A miniature paint sprayer that gives a controlled application of thinned paint.

All in one track

Model track that includes simulated roadbed or ballast. Also called combination or "click" track.

Alternating Current (AC)

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.

AREA

The American Railway Engineering Association is the prototype railroad organization which establishes right of-way material specifications and track construction standards.

Articulated

A steam locomotive with two engines (i.e., cylinders, rods, and wheels) under one boiler.

Automatic reversing

A circuit that senses short circuits and reverses the polarity. Used commonly with reversing loops, wyes, and turntables.