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4 foot

A commonly used term for "between the rails" (UK standard gauge being 4 foot 8 inches).

6 foot

Commonly used term for the space between two adjacent tracks.

7 year overhaul

Steam locomotives working on the National Network are required to undergo thorough boiler examinations every 7 years.

10 year overhaul

Steam locomotives working on independent railways are usually required to undergo thorough boiler examinations every 10 years, although this may sometimes be extended.

A4 Class

The A4 Class locomotive such as Mallard and Sir Nigel Gresley.

Absolute-Block Working

A system of train operation in which only one train at a time is allowed inside a block section. (see also permissive-block working).

Abt System

A rack-and-pinion system where the rack has two closely-adjacent staggered rows of teeth.

Adhesion factor

The ability of a locomotive to pull a heavy train without slipping.

Adriatic Locomotive

A locomotive which according to the Whyte classification is of the 2-6-4 wheel arrangement.

Advance direction (or in advance)

Forward from the location, in the normal direction of travel, for the line in question.

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".

Air bags

Slang term for the flexible pipes used to connect air brake systems between vehicles.

Air brakes

Each vehicle of an air braked train will have a cylinder with pressurised 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 pressurised. The train pipe runs the length of the train thus providing a fail-safe system should the train part.

Airfield Line

Marketing name for the Coventry Steam Railway Centre, a standard gauge railway line in Warwickshire built on a "greenfield" site.

Allan straight-link motion

A valve gear combining the features of Stephenson and Gooch valve gear, but where the expansion link and radius link move up or down together in opposite directions to obtain reverse. The name arises from the fact that the expansion link is straight in this valve gear instead of curved as in other valve gears.

Allegheny Locomotive

+A locomotive which according to the Whyte classification is of the 2-6+6-2 wheel arrangement.

American Wheel Arrangement

A locomotive which according to the Whyte classification is of the 4-4-0 wheel arrangement.

This was a light, quick and powerful locomotive designed by Rogers of Pattison, New Jersey, in 1855. They proved very popular with the railroads and were built by a number of manufacturers. More than 25000 were sold between 1855 and 1890.

Annett's Lock

A device on a ground frame which prevents the points levers on that frame from being reversed unless first unlocked by a special key. Once unlocked, the key cannot be removed until the points levers are returned to normal. A similar device, operated by the same key, is situated in the main signal-box, but in this case the signals protecting the area operated by the ground frame are locked.

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 flat 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.

Atlantic Locomotive

A locomotive which according to the Whyte classification is of the 4-4-2 wheel arrangement.

Atmospheric railway

A long-ago-obsolete system of railway motive power where traction is achieved by special vehicles which have a piston hanging underneath their bodywork fitting into a continuous tube located between the running rails. These tubes have a continuous slot along their upper surface which is sealed by a leather flap. Air is sucked out of the tube by stationary pumping engines in such a way that atmospheric air on one side of the piston drives the vehicle along against the vacuum on the other side of the piston.


A locomotive-powered train which may be driven with the locomotive either hauling or propelling one or more coaches. When propelling coaches, the train is driven from a special compartment at the front of the leading coach, where some of the driving controls are duplicated.

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 (ATC)

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; 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.

Auxiliary Steam Valve

Controls the supply of exhaust steam to the exhaust steam injector. See Injector for diagram and more details.

Avoiding line

A railway route which diverges from a main route and rejoins it again at another location after avoiding an area of congestion.


A system of electric motor suspension in which the weight of the whole motor is taken on the axle which is being driven.


This provides the bearing between the axle and locomotive frame. These were usually simple bearings designed to take the load (for driving wheels) of the alternating piston thrusts as well as the weight of steam locomotive. They were mounted in vertical slides within the frames and sprung to allow for uneven track. Relying upon a simple film of oil for lubrication, it was not uncommon for an axle box to overheat and seize (hence the term "hot box"). Later designed steam locomotives and diesels use roller bearings.

Axle Weight

The weight carried on any axle of a train. The maximum allowed would be limited by the track design, bridges and other structures on the line.