![Locomotive_trailing_truck](https://upload.wikimedia.org/wikipedia/commons/3/38/Locomotive_trailing_truck.png)
On a
steam locomotive
A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomot ...
, a trailing wheel or trailing axle is generally an unpowered
wheel
A wheel is a circular component that is intended to rotate on an axle Bearing (mechanical), bearing. The wheel is one of the key components of the wheel and axle which is one of the Simple machine, six simple machines. Wheels, in conjunction wi ...
or
axle
An axle or axletree 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 the vehicle, with the wheels rotating around the axle. In the former case, bearing ...
(
wheelset) located behind the driving wheels. The axle of the trailing wheels is usually located in a trailing
truck
A truck or lorry is a motor vehicle designed to transport cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame construction ...
. On some large locomotives, a
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 the lead truck on the tender. A rocking idler gear permits it to be put into operation by the drive ...
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.
[Alfred W Bruce.(1952)'' The Steam Locomotive in America - Its Development in the Twentieth Century'' New York, U.S.A. : Bonanza Books. p239-40, 256-57]
![Delta trailing truck Commonwealth Steel Co](https://upload.wikimedia.org/wikipedia/commons/5/54/Delta_trailing_truck_Commonwealth_Steel_Co.jpg)
In the
Whyte notation
Whyte notation is a classification method for steam locomotives, and some internal combustion locomotives and electric locomotives, by wheel arrangement. It was devised by Frederick Methvan Whyte, and came into use in the early twentieth ce ...
, trailing wheels are designated by the last numbers in the series. For example, the
2-8-2
Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of two leading wheels on one axle, usually in a leading truck, eight powered and coupled driving wheels on four axles and two trailing wheel ...
Mikado type locomotive had two
leading wheels
The leading wheel or leading axle or pilot wheel of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located on a leading truck. Leading wheels are used ...
, eight
driving wheels
On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons (or turbine, in the case of a steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled tog ...
, and two trailing wheels. Some locomotives such as the
4-4-0
4-4-0 is a locomotive type with a classification that uses the Whyte notation for the classification of steam locomotives by wheel arrangement and represents the arrangement: four leading wheels on two axles (usually in a leading bogie), four po ...
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 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
The 2-6-6-6 (in Whyte notation) is an articulated locomotive type with two 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 L ...
Allegheny type and the
Pennsylvania Railroad
The Pennsylvania Railroad (reporting mark PRR), legal name The Pennsylvania Railroad Company also known as the "Pennsy", was an American Class I railroad that was established in 1846 and headquartered in Philadelphia, Pennsylvania. It was named ...
's
6-8-6
Under the Whyte notation for the classification of steam locomotives by wheel arrangement, represents the arrangement of six unpowered leading wheels arranged into a three-axle leading truck, eight powered driving wheels, and six unpowered trail ...
steam turbine
A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam turbin ...
and
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:
* UIC classification: ...
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. The concept was first used in France in 18 ...
s, as well as numerous
Mason Bogie
Mason Bogie locomotives (also known as Mason Fairlie locomotives) are a type of articulated steam locomotive suited for sharp curves and uneven track, once commonly used on narrow gauge railways in the United States of America. The design is a de ...
locomotives.
In the
UIC classification
The UIC classification of locomotive axle arrangements, sometimes known as the German classification''The Railway Data File''. Leicester: Silverdale, 2000. p. 52. . or German system,Kalla-Bishop P.M. & Greggio, Luciano, ''Steam Locomotives'', Cre ...
system, the number of axles rather than the number of wheels is counted.
See also
*
AAR wheel arrangement
The AAR wheel arrangement system is a method of classifying locomotive (or unit) wheel arrangements that was developed by the Association of American Railroads. Essentially a simplification of the European UIC classification, it is widely used in ...
*
Steam locomotive components
__NOTOC__
This article is a glossary of the main components found on a typical steam locomotive.
The diagram, which is not to scale, is a composite of various designs in the late steam era. Some components shown are not the same, or are not pre ...
*
UIC classification of locomotive axle arrangements
The UIC classification of locomotive axle arrangements, sometimes known as the German classification''The Railway Data File''. Leicester: Silverdale, 2000. p. 52. . or German system,Kalla-Bishop P.M. & Greggio, Luciano, ''Steam Locomotives'', Cre ...
*
Whyte notation
Whyte notation is a classification method for steam locomotives, and some internal combustion locomotives and electric locomotives, by wheel arrangement. It was devised by Frederick Methvan Whyte, and came into use in the early twentieth ce ...
References
{{Locomotive running gear
Train wheels