Train Terminology

There are some rather obscure (to the uninitiated) terms used for certain parts and types, as well as configurations of steam trains. Here are some of the more frequently-used ones.
Picture of Mallard steam locomotive

Wheel Configuration Systems - Whyte (e.g. 2-6-2) and UIC (e.g. 1C1)
There are three main classes of wheel: pilot, or leading wheels; driver or driving wheels; and trailing wheels. The Whyte system identifies numbers of each type of wheel. In the picture at right, there are four leading, un-driven wheels, then six driven wheels, then 2 trailing wheels, making it a 4-6-2. In UIC it would be classified as 2C1.
There are several other symbols used in this system to indicate various things, but these will not be covered as Whyte terminology is used on this website.
Classes of Locomotives
The various classes of locomotives are often named after famous people, places or events, but they also have a classification based on their strength and usual usage. For example, a 9F class has a high tractive effort (9 being the highest) and is designed to pull freight (as distinct from P for passenger).
Flanges (on wheels)
Train wheels generally have a flange on the inner edge. Contrary to what many believe, the flanges rarely come into contact with the line, and are merely an added safety measure. The (almost) horizontal surface which contacts the track usually (but not always) has a slight taper to its inner-edge so the wheel has a conical shape. This allows the train flexibility as the body sways during travel, by the contact point moving slightly from side to side.
Blind Wheels
These are driving wheels with no flange. This is done on some locos with a large number of driving wheels, to improve their ability to navigate turns.
Driven Wheels, or Driving Wheels
Driving wheels are usually the largest of the wheels on a loco, and are driven by the engine's pistons and connected to each other in groups by side-rods, or connecting-rods.
Articulated Locos
These have their driving wheels connected into several groups, instead of one group, by side-rods.
Tractive Effort (locomotive)
This is the loco's ability to move a load, and is governed by several factors: torque applied to the driving wheels; wheel diameter; friction between the driving wheels and the rails; and the weight that the loco applies to the driving wheels.
Double Header Train
When a train has a very heavy load to move or a sharp incline to climb (or descend) a second loco is sometimes coupled to it. This may be directly behind the driver loco, or at the rear. Because there is no way to link the controls between two steam locos, a system of whistles is used (or was - radios are used now) for communication between the two crews. It's important for their actions to be synchronised as great damage - even derailment - could result if for instance the driver loco was braking while the assist loco was pushing.
Reciprocating Engine
This simply means an engine that uses pistons, inside a cylinder, just as motor vehicles do. A flywheel provides the inertia required to carry the piston beyond its fully-extended position.
Banker (locomotive)
A "banker" or helper loco is used to assist another to move a heavy load or climb a steep slope. It is usually attached at the rear of the train, as this better allows it to assist with moving and with braking.
Tank Engine
A tank engine is one that carries its water on board, without the use of a tender.
Picture of a caboose, or guards van

Caboose, Brake Van or Guard's Van
A wagon at the rear of a train with a brake that can be applied by the Guard to reduce the pressure on the leading loco and wagons when braking or going downhill. The Guard has added functions too, such as alerting the driver of danger approaching from the rear, or confirming when it's safe to start moving a passenger train.
Bogie
A bogie is a set of wheels (usually four, but not always) assembled on a trolley, which becomes a part of a locomotive or carriage. Most locos and carriages have one bogie at each end. The brake-van at right has one four-wheel bogie. Four-wheel bogies are the most commonly used, but some may have up to ten wheels.
Tender
A tender or coal-car is a special rail vehicle hauled by a steam locomotive containing its fuel and water. A locomotive that pulls a tender is called a tender locomotive. Locomotives that do not have tenders and carry all their fuel and water on board are called tank engines. Large steam locomotives are usually semi-permanently coupled by a drawbar to a tender. Some countries use water troughs (track pans in the US)) on some lines to allow locomotives to replenish their water supply while moving.
Valve Gear
The mechanism that controls the opening and closing of the engine's ports to allow steam to pass into the cylinders, driving the train.
Walschaerts Valve
The valve gear mechanism that is used in the majority of British steam locomotives, replacing the Stephenson valve gear. It is mounted externally to the engine, and thus lends itself better to articulated engines.
Water Trough
Steam locos use large amounts of water, and to reduce time water troughs are installed, usually beside the track but sometimes between the rails, on some lines. The fireman in the tender lowers a scoop, which scoops up water from the trough while moving, then raises the scoop to continue on.
Water Scoop
A water scoop is sometimes fitted under the tender (or the rear water tank in the case of a large tank engine). The fireman remotely lowers the scoop into the trough and the speed of the engine forces the water up into the tank, and the scoop is raised once it's full.
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