On a steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

, a trailing wheel or trailing axle is generally an unpowered wheel

Wheel

A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...

 or axle

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

 (wheelset) located behind the driving wheels. The axle of the trailing wheels was usually located on a trailing truck

Bogie

A bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...

. On some large locomotives, a Booster engine

Booster engine

A booster engine for steam locomotives is a small two-cylinder steam engine back-gear-connected to the trailing truck axle on the locomotive or, if none, the lead truck on the tender. A rocking idler gear permits it to be put into operation by the engineer...

 was mounted on the trailing truck to provide extra tractive effort when starting a heavy train and at low speeds on gradients.

Trailing wheels were used in some early locomotives but fell out of favor for a time
during the latter 19th century. As demand for more powerful locomotives increased, trailing wheels began to be used to support the crew cab and rear firebox area.

Trailing wheels first appeared on American locomotives between 1890 and 1895, but their axle worked in rigid pedestals. It enabled boilers to be lowered, since the top of the main frames was dropped down behind the driving wheels and under the firebox. The firebox could also be longer and wider, increasing the heating surface area and steam generation capacity of the boiler, and therefore its power. The concept was soon improved to provide radial lateral movement by placing the pair of trailing wheels and their axle in a fabricated sub-frame or truck, usually with outside bearings as they gave the best lateral riding stability. One-piece cast-steel trailer trucks were developed about 1915, to provide the additional strength for a booster engine to be fitted to the trailing axle. Finally, about 1921 the Delta trailing truck was developed with an inverted-rocker centering device at the rear ends of the truck frame. Delta trucks were soon enlarged to carry four trailing wheels, and later six.
In the Whyte notation

Whyte notation

The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early twentieth century encouraged by an editorial in American Engineer and Railroad Journal...

, trailing wheels are designated by the last numbers in the series. For example, the 2-8-2

2-8-2

Under the Whyte notation for the classification of steam locomotives, 2-8-2 represents the wheel arrangement of two leading wheels on one axle , eight powered and coupled driving wheels on four axles, and two trailing wheels on one axle...

 Mikado type locomotive had two leading wheels, eight driving wheels, and two trailing wheels. Some locomotives such as the 4-4-0

4-4-0

Under the Whyte notation for the classification of steam locomotives, 4-4-0 represents the wheel arrangement of four leading wheels on two axles , four powered and coupled driving wheels on two axles, and no trailing wheels...

 American type had no trailing wheels and were designated with a zero in the final place.

In the Whyte notation the number designates the number of wheels rather than the number of axles, thus the final 2 in the Mikado's 2-8-2 refers to two wheels (one axle) while the Northern type's

4-8-4

Under the Whyte notation classification of steam locomotives, 4-8-4 represents the wheel arrangement of four leading wheels on two axles , eight powered and coupled driving wheels on four axles, and four trailing wheels on two axles .Other equivalent classifications are:UIC classification: 2D2...

 4-8-4 designation refers to four wheels (two axles).

The highest number of trailing wheels on a single locomotive is six as seen on 2-6-6-6

2-6-6-6

The 2-6-6-6 is an articulated locomotive type with 2 leading wheels, two sets of six driving wheels and six trailing wheels. Only two classes of the 2-6-6-6 type were built. One was the "Allegheny" class, built by the Lima Locomotive Works. The name comes from the locomotive's first service with...

 Allegheny type and the Pennsylvania Railroad

Pennsylvania Railroad

The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....

's 6-8-6

6-8-6

In Whyte notation, a 6-8-6 is a steam locomotive with:* six unpowered leading wheels arranged into a three-axle leading truck,* eight powered driving wheels, and* six unpowered trailing wheels arranged into a three-axle trailing truck....

 steam turbine

Steam turbine locomotive

A steam turbine locomotive is a steam locomotive which transmits steam power to the wheels via a steam turbine. Numerous attempts at this type of locomotive were made, mostly without success...

 and 6-4-4-6

6-4-4-6

A 6-4-4-6 steam locomotive, in the Whyte notation for describing locomotive wheel arrangements, is one with six leading wheels, two sets of four driving wheels, and six trailing wheels.Other equivalent classifications are:...

 duplex locomotive

Duplex locomotive

A duplex locomotive is a steam locomotive that divides the driving force on its wheels by using two pairs of cylinders rigidly mounted to a single locomotive frame; it is not an articulated locomotive...

s, as well as numerous Mason Bogie

Mason Bogie

The Mason Bogie is a type of articulated steam locomotive suited for sharp curves and uneven track, once commonly used on narrow gauge lines in the United States...

 locomotives.

In the UIC classification

UIC classification

The UIC classification of locomotive axle arrangements describes the wheel arrangement of locomotives, multiple units and trams. It is set out in the International Union of Railways "Leaflet 650 - Standard designation of axle arrangement on locomotives and multiple-unit sets". It is used in much...

 system, the number of axles rather than the number of wheels is counted.

See also

• Steam locomotive nomenclature
• AAR wheel arrangement
  AAR wheel arrangement
  The AAR wheel arrangement system is a method of classifying locomotive wheel arrangements that was developed by the Association of American Railroads. It is essentially a simplification of the European UIC classification, and it is widely used in North America to describe diesel and electric...
  
  
• UIC classification
  UIC classification
  The UIC classification of locomotive axle arrangements describes the wheel arrangement of locomotives, multiple units and trams. It is set out in the International Union of Railways "Leaflet 650 - Standard designation of axle arrangement on locomotives and multiple-unit sets". It is used in much...
  
  
• Whyte notation
  Whyte notation
  The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early twentieth century encouraged by an editorial in American Engineer and Railroad Journal...