{{Short description|Wheel designed for railway tracks}} {{Distinguish|Training wheels}} {{Use dmy dates|date=May 2025}} {{Use British English|date=May 2025}} [[File:Rollingstock axle.jpg|thumb|250px|Both wheels of a [[Wheelset (rail transport)|wheelset]] rotate at the same rate because the wheels are fixed rigidly on the axle]]
A '''train wheel''' or '''rail wheel''' is a type of [[wheel]] specially designed for use on [[Track (rail transport)|railway track]]s. The wheel acts as a rolling component, typically [[press fit]]ted on to an [[axle]] and mounted directly on a [[Railroad car|railway carriage]] or [[locomotive]], or indirectly on a [[bogie]] ({{abbr|CwthE|Commonwealth English}}) or truck ({{abbr|NAmE|North American English}}). The powered wheels under the locomotive are called [[driving wheel]]s. Wheels are initially [[casting|cast]] or [[forging|forged]] and then heat-treated to have a specific hardness.<ref>{{Cite book|title=Wheel–Rail Interface Handbook |isbn=9781845694128 |url=https://www.sciencedirect.com/book/9781845694128/wheel-rail-interface-handbook|access-date=2020-10-29 |last1=Lewis |first1=R. |last2=Olofsson |first2=U. |date=25 September 2009 |publisher=Elsevier Science }}</ref> New wheels are machined using a [[lathe]] to a standardised shape, called a profile. All wheel profiles are regularly checked to ensure proper [[wheel-rail interface|interaction between the wheel and the rail]].{{Citation needed|date=April 2025}} Incorrectly profiled wheels and worn wheels can increase [[rolling resistance]], reduce [[energy efficiency in transport|energy efficiency]] and may even cause a [[derailment]].<ref>{{Cite book|title=Wheel-rail interface handbook|date=2009|publisher=CRC Press|last1=Lewis |first1=Roger |last2=Olofsson |first2=Ulf|isbn=978-1-61583-153-1|location=Boca Raton, Florida|oclc=500906475}}</ref> The [[International Union of Railways]] has defined a standard wheel diameter of {{Convert|920|mm|in|abbr=on}}, although smaller sizes are used in some [[rapid transit]] railway systems and on [[Rolling highway|ro-ro carriages]].<ref>{{Cite book|last=Licitra|first=Gaetano|url=https://books.google.com/books?id=a9bKBQAAQBAJ&dq=train+wheels+rim+monoblock&pg=PA83|title=Noise Mapping in the EU: Models and Procedures|date=2012-09-06|publisher=CRC Press|isbn=978-0-203-84812-8|language=en}}</ref>
==Wheel geometry and flange== {{See also|Derailment|Adhesion railway|Hunting oscillation}} [[File:Drawing -- railway wheel and rail profiles.png|thumb|250px|A railway wheel's [[Wheelset (rail transport)#Conical wheel-tread|tread]] and [[Train wheel#Wheel geometry and flange|flange]] and its relationship to the load-bearing rail]]
The running surface ("tread" or "rim") of most train wheels is [[Cone|conical]], which serves as the primary means of keeping the train aligned centrally on the [[Track (rail transport)|track]] while in motion. On curves, the [[Wheelset (rail transport)|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.<ref name=":0">{{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}}</ref> 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.<ref>{{Cite AV media |people=Richard Feynman |url=https://www.youtube.com/watch?v=y7h4OtFDnYE |title=Feynman: how the train stays on the track |date=1983 |work=Fun to Imagine |publisher=BBC TV |via=YouTube.com }}</ref>
==Wheel arrangement==
{{Main|Wheel arrangement}}The 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 (rail transport)|wheelset]].<ref name=":0" />
==Wheels for road–rail vehicles== [[File:Land cruiser hyrail conversion.jpg|thumb|left|The small rail wheels fitted to road–rail vehicles allow them to be stowed away when the vehicle is in road-going mode.]] Wheels used for [[road–rail vehicle]]s are normally smaller than those found on other types of [[rolling stock]], 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.<ref name=":1">{{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}}</ref> Some wheels, however, are made of two parts: the wheel core, and a tyre ({{abbr|CwthE|Commonwealth English}}) or [[tire]] ({{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 [[spoke]]d construction. In modern times the tyre is invariably made from [[steel]], which is stronger than the [[cast iron]] 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]], between the wheel and tyre. Failure of this type of wheel was one of the causes leading to the [[Eschede train disaster#Wheel design|Eschede high-speed train crash]].<ref name=":1"/>
==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 [[Railway brake|brake shoes]] (or brake blocks) are applied directly to the wheel surface, which generates immense amounts of [[thermal energy]]: under normal operation, a wheel may reach a tread temperature of {{Convert|550|C|F|abbr=on}}.<ref>{{Cite journal|last1=Peters|first1=Carsten J.|last2=Eifler|first2=Dietmar|date=2009-11-01|title=Influence of Service Temperatures on the Fatigue Behaviour of Railway Wheel and Tyre Steels*|url=https://www.degruyter.com/document/doi/10.3139/120.110094/html|journal=Materials Testing|language=en|volume=51|issue=11–12|pages=748–754|doi=10.3139/120.110094|bibcode=2009MTest..51..748P |s2cid=135684020|issn=2195-8572|url-access=subscription}}</ref> Under severe braking conditions, the generated thermal energy can contribute to [[thermal shock]] or alteration of the wheel's mechanical properties. Ultimately, acute thermal loading leads to a phenomenon called [[spall]]ing. Alternatively, severe braking or [[Slippery rail|low adhesion]] may stop the rotation of the wheels while the vehicle is still moving, which may cause a [[flat spot]] on the [[Wheel–rail interface|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.<ref>{{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.}}</ref>
== Guide wheel == [[Rubber-tyred metro]]s with a central [[Guide rail#Railway guide rail|guide rail]], such as the [[Busan Metro Line 4|Busan Metro]], [[Lille Metro]] and the [[Sapporo Municipal Subway]] as well as [[rubber-tyred tram]]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]] [[Guide rail#Railway guide rail|guide rail]] (green) and the tram's '''guide wheels''' (red). Right: cross section of the guide [[Rail profile|rail]] and '''guide wheel''' of the Bombardier's [[Bombardier Guided Light Transit|GLT]] }}
==See also== * [[Adhesive weight]] * [[Cycloid]] and [[trochoid]] * [[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]]|date=1998-03-26|access-date=2015-01-17}}
{{Locomotive running gear}} {{Authority control}}
[[Category:Train wheels| ]]