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A train wheel or rail wheel is a type of
wheel A wheel is a rotating component (typically circular in shape) that is intended to turn 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 machin ...
specially designed for use on
railway track Railway track ( and UIC terminology) or railroad track (), also known as permanent way () or "P way" ( and Indian English), is the structure on a railway or railroad consisting of the rails, fasteners, sleepers ( railroad ties in American ...
s. The wheel acts as a rolling component, typically
press fit An interference fit, also known as a pressed fit or friction fit, is a form of fastening between two tightfitting mating parts that produces a joint which is held together by friction after the parts are pushed together. Depending on the amoun ...
ted on to an
axle An axle or axletree is a central shaft for a rotation, rotating wheel and axle, 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 ...
and mounted directly on a
railway carriage A railroad car, railcar ( American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and UIC), also called a train car, train wagon, train carriage or train truc ...
or
locomotive A locomotive is a rail transport, rail vehicle that provides the motive power for a train. Traditionally, locomotives pulled trains from the front. However, Push–pull train, push–pull operation has become common, and in the pursuit for ...
, or indirectly on a
bogie A bogie ( ) (or truck in North American English) comprises two or more Wheelset (rail transport), wheelsets (two Railroad wheel, wheels on an axle), in a frame, attached under a vehicle by a pivot. Bogies take various forms in various modes ...
() or truck (). The powered wheels under the locomotive are called
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 ...
. Wheels are initially
cast Cast may refer to: Music * Cast (band), an English alternative rock band * Cast (Mexican band), a progressive Mexican rock band * The Cast, a Scottish musical duo: Mairi Campbell and Dave Francis * ''Cast'', a 2012 album by Trespassers William ...
or
forged Forging is a manufacturing process involving the shaping of metal using localized compression (physics), compressive forces. The blows are delivered with a hammer (often a power hammer) or a die (manufacturing), die. Forging is often classif ...
and then heat-treated to have a specific hardness. New wheels are machined using a
lathe A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, threading and turning, with tools that are applied to the w ...
to a standardised shape, called a profile. All wheel profiles are regularly checked to ensure proper interaction between the wheel and the rail. Incorrectly profiled wheels and worn wheels can increase
rolling resistance Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the Motion (physics), motion when a body (such as a ball, tire, or wheel) Rolling, rolls on a surface. It is mainly caused by Plasticity (physics), non- ...
, reduce
energy efficiency Energy efficiency may refer to: * Energy efficiency (physics), the ratio between the useful output and input of an energy conversion process ** Electrical efficiency, useful power output per electrical power consumed ** Mechanical efficiency, a rat ...
and may even cause a
derailment In rail transport, a derailment is a type of train wreck that occurs when a rail vehicle such as a train comes off its rails. Although many derailments are minor, all result in temporary disruption of the proper operation of the railway sys ...
. The
International Union of Railways The International Union of Railways (, UIC) is an international rail transport industry body based in Paris. History The railways of Europe had originated during the nineteenth century as many separate concerns across numerous nations; this le ...
has defined a standard wheel diameter of , although smaller sizes are used in some
rapid transit Rapid transit or mass rapid transit (MRT) or heavy rail, commonly referred to as metro, is a type of high-capacity public transport that is generally built in urban areas. A grade separation, grade separated rapid transit line below ground su ...
railway systems and on ro-ro carriages.


Wheel geometry and flange

The running surface ("tread" or "rim") of most train wheels is
conical In geometry, a cone is a three-dimensional figure that tapers smoothly from a flat base (typically a circle) to a point not contained in the base, called the ''apex'' or '' vertex''. A cone is formed by a set of line segments, half-lines, ...
, which serves as the primary means of keeping the train aligned centrally on the track while in motion. On curves, the wheelset becomes "self-steering" because of this coning: when one wheel is pushed closer to the adjacent (outer) rail, it presents a bigger circumference to the rail than when centred on the track. Simultaneously, the inner wheel presents a smaller circumference to its rail. The difference between the distances travelled by each wheel for each rotation of the axle causes the wheelset to follow the curve of the track. {{Cite web, title=Book: The Contact Patch, url=https://the-contact-patch.com/book/rail/r1610-the-wheelset, access-date=2020-10-29, website=the-contact-patch.com, language=en If a strong sideways force is experienced – for example, when the radius of a curve is smaller than normal or there are defects in the track alignment – the wheelset will depart from its equilibrium. That is when a projection on the inner side of each wheel, called a flange, constrains the wheelset from moving further and derailing.


Wheel arrangement

{{Main, Wheel arrangementThe number of wheels per locomotive or car varies in both size and number to accommodate the needs of the vehicle. Regardless of these factors, pairs of identically sized wheels are always affixed to a straight axle as a singular unit, called a wheelset.


Wheels for road–rail vehicles

Wheels used for
road–rail vehicle A road–rail vehicle or a rail–road vehicle is a dual-mode vehicle which can operate both on rail tracks and roads. They are also known as two-way vehicles (), hi-rail (from ''highway'' and ''railway'', or variations such as high-rail, HiRa ...
s are normally smaller than those found on other types of
rolling stock The term rolling stock in the rail transport industry refers to railway vehicles, including both powered and unpowered vehicles: for example, locomotives, Railroad car#Freight cars, freight and Passenger railroad car, passenger cars (or coaches) ...
, such as locomotives or carriages, because the wheel has to be stowed clear of the ground when the vehicle is in road-going mode. Such wheels can be as small as {{convert, 245, mm, in, 2, abbr=on in diameter.{{Citation needed, date=March 2025 {{clear


Railway wheel and tire

{{main, Railway tire Modern railway wheels are usually machined from a single casting, also known as monobloc wheels.{{Cite book, last1=Milne, first1=Ian, url=https://books.google.com/books?id=hpc60yCsTPcC&dq=monoblock+train+wheels+advantage&pg=PA56, title=Comprehensive Structural Integrity, last2=Ritchie, first2=R. O., last3=Karihaloo, first3=B. L., date=2003-07-25, publisher=Elsevier, isbn=978-0-08-049073-1, language=en Some wheels, however, are made of two parts: the wheel core, and a tyre ({{abbr, CwthE, Commonwealth English) or
tire A tire (North American English) or tyre (Commonwealth English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineeri ...
({{abbr, NAmE, North American English) around the perimeter. Separate tyres have been a component of some rolling stock, their purpose being to provide a replaceable wearing element – an important factor in the steam locomotive era with their costly spoked construction. In modern times the tyre is invariably made from
steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
, which is stronger than the
cast iron Cast iron is a class of iron–carbon alloys with a carbon content of more than 2% and silicon content around 1–3%. Its usefulness derives from its relatively low melting temperature. The alloying elements determine the form in which its car ...
of earlier eras. It is typically heated and pressed on to the wheel before it cools and shrinks. ''Resilient rail wheels'' have a resilient material, such as
rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Types of polyisoprene ...
, between the wheel and tyre. Failure of this type of wheel was one of the causes leading to the Eschede high-speed train crash.{{Cite book, last1=Milne, first1=Ian, url=https://books.google.com/books?id=hpc60yCsTPcC&dq=monoblock+train+wheels+advantage&pg=PA56, title=Comprehensive Structural Integrity, last2=Ritchie, first2=R. O., last3=Karihaloo, first3=B. L., date=2003-07-25, publisher=Elsevier, isbn=978-0-08-049073-1, language=en


Causes of damage

The most common cause of wheel damage is severe braking, including sudden braking, braking on steep gradients and braking with heavy loads. The
brake shoes A railway brake is a type of brake used on the cars of railway trains to enable deceleration, control acceleration (downhill) or to keep them immobile when parked. While the basic principle is similar to that on road vehicle usage, operational ...
(or blocks) are applied directly to the wheel surface, which generates immense amounts of
thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...
: under normal operation, a wheel may reach a tread temperature of {{Convert, 550, C, F, abbr=on. Under severe braking conditions, the generated thermal energy can contribute to
thermal shock Thermal shock is a phenomenon characterized by a rapid change in temperature that results in a transient mechanical load on an object. The load is caused by the differential expansion of different parts of the object due to the temperature chang ...
or alteration of the wheel's mechanical properties. Ultimately, acute thermal loading leads to a phenomenon called
spall Spall are fragments of a material that are broken off a larger solid body. It can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ba ...
ing. Alternatively, severe braking or low adhesion may stop the rotation of the wheels while the vehicle is still moving, which may cause a
flat spot A flat spot, or wheel flat, also called spalling or shelling, is a fault in railroad wheel shape. A flat spot occurs when a rail vehicle's wheelset stops rotating while the train is still in motion, causing part of the wheel to ablate against ...
on the wheel-rail interface and localised heat damage.{{Citation needed, date=March 2025 Modern railway wheels are manufactured reasonably thickly to provide an allowance of wear material, since worn wheel profiles or wheels with a flat spot can be machined on a wheel lathe if there is sufficient thickness of material remaining.{{Cite book, last=Nielsen, first=J., title=Wheel–Rail Interface Handbook, chapter=Out-of-round railway wheels , date=2009-01-01, chapter-url=http://www.sciencedirect.com/science/article/pii/B9781845694128500082, pages=245–279, editor-last=Lewis, editor-first=R., publisher=Woodhead Publishing, language=en, doi=10.1533/9781845696788.1.245, isbn=978-1-84569-412-8, access-date=2020-10-29, editor2-last=Olofsson, editor2-first=U.


Guide wheel

Rubber-tyred metro A rubber-tyred metro or rubber-tired metro is a form of rapid transit system that uses a mix of road transport, road and rail transport, rail technology. The vehicles have wheels with rubber tires that run on a roll way inside guide bars for tr ...
s with a central
guide rail A guide rail is a device or mechanism to direct products, vehicles or other objects through a channel, conveyor, roadway or rail system. Several types of guide rails exist and may be associated with: * Factory or production line conveyors * Pow ...
, such as the Busan Metro,
Lille Metro The Lille Metro () is a driverless light metro system located in Lille, France. It commenced operations on 25 April 1983 and was the first system to implement the Véhicule Automatique Léger (VAL, ) technology. While sometimes cited as the ...
and the
Sapporo Municipal Subway The is a mostly-underground rubber-tyred rapid transit system in Sapporo, Hokkaido, Japan. Operated by the Sapporo City Transportation Bureau, it is the only subway system on the island of Hokkaido. Lines The system consists of three lines: ...
as well as
rubber-tyred tram A rubber-tyred tram (also known as tramway on tyres, ) is a development of the guided bus in which a vehicle is guided by a fixed rail in the road surface and draws current from overhead electric wires (either via pantograph or trolley poles) ...
s have guide wheels.{{Citation needed, date=March 2025 {{multiple image , align = center , image1 = TranslohrGuideRail.svg , width1 = 200 , alt1 = , caption1 = , image2 = TVRGeleiding.svg , width2 = 200 , alt2 = , caption2 = , footer = Left: diagram of the
Translohr Translohr is the name of a rubber-tyred tramway technology, originally developed by Lohr Industrie of France and now run by a consortium of Alstom Transport and Fonds stratégique d'investissement (FSI) as ''newTL,'' which took over from ...
guide rail A guide rail is a device or mechanism to direct products, vehicles or other objects through a channel, conveyor, roadway or rail system. Several types of guide rails exist and may be associated with: * Factory or production line conveyors * Pow ...
(green) and the tram's guide wheels (red). Right: cross section of the guide
rail Rail or rails may refer to: Rail transport *Rail transport and related matters *Railway track or railway lines, the running surface of a railway Arts and media Film * ''Rails'' (film), a 1929 Italian film by Mario Camerini * ''Rail'' (1967 fil ...
and guide wheel of the Bombardier's GLT


See also

*
Adhesive weight Adhesive weight is the weight on the driving wheels of a locomotive, which determines the frictional grip between wheels and rail, and hence the drawbar pull which a locomotive can exert. See also *Factor of adhesion *Tractive effort References ...
*
Cycloid In geometry, a cycloid is the curve traced by a point on a circle as it Rolling, rolls along a Line (geometry), straight line without slipping. A cycloid is a specific form of trochoid and is an example of a roulette (curve), roulette, a curve g ...
and
trochoid In geometry, a trochoid () is a roulette curve formed by a circle rolling along a line. It is the curve traced out by a point fixed to a circle (where the point may be on, inside, or outside the circle) as it rolls along a straight line. If the ...
* Wheelset (rail transport)#Conical wheel-tread


References

{{Reflist


External links

* {{Cite web, title=APTA PR-CS-RP-003-98 Recommended Practice for Developing a Clearance Diagram for Passenger Equipment 5.3.2.1 Design tolerances, url=http://www.apta.com/resources/standards/Documents/APTA-PR-CS-RP-003-98.pdf, website=APTA.com, publisher=
American Public Transportation Association The American Public Transportation Association (APTA) is a nonprofit group of approximately 1,500 public and private sector member organizations that promotes and advocates for the interests of the public transportation industry in the United ...
, date=1998-03-26, access-date=2015-01-17 {{Locomotive running gear {{Authority control