- Absolute signal: a block signal whose most restrictive indication is stop. An absolute signal is identified by having no number plate or by an A-plate affixed to the mast or supporting structure. Proceeding beyond an absolute signal that is displaying a stop indication is prohibited unless authorized by a dispatcher.
- Adhesion: the frictional grip between wheel and rail.
- Adhesive weight: the weight on the driving wheels of a pends the frictional grip between wheels and rail and hence the drawbar pull which a locomotive can exert.
- Air brake: power breaking system with compressed air as the operating medium.
- air cushion: type of spring used in some modern passenger car suspension systems with air as the operating medium.
- Alternator : a machine which converts mechanical energy to electrical energy and generates alternating current.
- Alco: American Locomotive Company - the second largest builder of steam locomotives in the U.S.
- Alerter: Similar to the Dead man's switch other than it does not require the operator's constant interaction. Instead, an alarm is sounded at a preset interval in which the operator must respond by pressing a button to reset the alarm and timer if no other controls are operated. If the operator does not respond within a preset time, the prime mover is automatically throttled back to idle and the brakes are automatically applied. May also be called a 'Watchdog'.
- All weather adhesion: the maximum adhesion that can be expected of a locomotive in all weather conditions.
- American: A steam locomotive with a 4-4-0 wheel arrangement.
- Angel Seat or Angel's Perch: (US) A term often used when referring to the second level seats on a Cupola style caboose.
- Angle cock: A valve affixed to each end of a piece of rolling stock that, when opened, admits compressed air to the brake pipe (or vents it to the atmosphere if air hose is detached).
- Annett's key (UK), Annett key (AU): A large key which locks levers or other items of signalling apparatus, serving as a portable form of interlocking. With the key removed from the lock, the lever or apparatus is locked in its position. When the key is turned in the lock, it cannot be removed.
- Anti-slide/skid: a device for detecting an automatically correcting wheel slide or skid during braking by a momentary reduction of braking force.
- Anti-slip: electrical circuit which detects driving wheel slip on diesel and electric locomotives. The difference in current taken by a particular traction motor when wheel slip occurs causes an illuminated warning to be givin to the engineer. In addition, an automatic reduction in motive power and/or partial application of locomotive brakes may be effected.
- Arch tubes: tubes connected to the water-space of the boiler provided in and across the firebox in order to add extra high-
- Armature the rotating part of direct current electrc motor or generator. Contains a number of coils, or warning, which rotate in a magnetic field and are connected to the commutator.
temperature heating surface. They also serve to support the brick arch or equivalent.
- Articulated locomotive: A steam locomotive with one or more engine units that can move relative to the main frame.
- Articulation the sharing of one truck (US) by adjacent ends of two rail vehicles.
- Ash-pan: a feature of a locomotive which has the some form and purpose as the domestic variety, ie, to collect the ashes which fall through the bars of the grate. The only significant difference is the size, measured in fleet rather then inches.
- Aspect: (UK) The indication displayed by a colour-light signal e.g. a yellow aspect
- Asynchronous: an alternating current electric motor whose speed varies with load and has no fixed relation to the frequency of the supply.
- Atlantic: A steam locomotive with a 4-4-2 wheel arrangement.
- A-unit (US): A Diesel locomotive equipped with a full cab and control stand, and hence capable of being the lead unit in a consist. An A-unit that has MU capability is able to control other A-units with MU capability or B-units.
- Auto brake A type of fail-safe system that uses air pressure to hold the brakes off so that in the event the air pressure is lost in the brake pipe the brakes will automatically apply.
- Auto-brake gauge: A gauge recording the application and pressure of an automatic braking system; usually repeated in the guard's van in historic rolling stock.
- Autocoach (UK): A passenger coach fitted with a driving cab and controls for use in an Autotrain (UK).
- Automatic Equipment Identification (AEI) (US): Automatic tracking system using RFID technology.
- Automatic Train Control (ATC)
- Automatic train operation (ATO)
- Automatic train protection (ATP)
- Automatic Warning System (UK): Refers to the specific form of limited cab signalling introduced in 1948 in the United Kingdom to help train drivers observe and obey warning signals.
- Autorack (also called auto carrier) (US): A specialized freight car for transporting automobiles. Car transporter wagon / Car transporter van (UK).
- Autotrain (UK): A branch line train consisting of a steam locomotive and passenger carriages that can be driven from either end by means of rodding to the regulator and an additional vacuum brake valve. The fireman remains with the locomotive and, when the driver is at the other end, the fireman controls the cut off and vacuum ejectors in addition to his usual duties. Also: Push-pull train, Motor train (UK).
- Auto Train (US): A passenger train service first operated by Auto-Train Corporation and then by Amtrak between Lorton, Virginia and Sanford, Florida that carries the passengers' automobiles aboard the same train in autoracks.
- Axle box the axle bearings of a locomotive are known as axleboxes. It is usually convenient to make them boxshaped to suit the guides and openings in the frames which should constrain movement in the horizontal plane but allow freedom vertically. see journal box below
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