Train wheel

{{See also|Derailment|Adhesion railway|Hunting oscillation}}

File:Drawing -- railway wheel and rail profiles.png and flange and its relationship to the load-bearing rail]]

The running surface ("tread" or "rim") of most train wheels is conical, 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.{{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 }}

{{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.

File:Land cruiser hyrail conversion.jpg

Wheels used for road–rail vehicles 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}}

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{{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 ({{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, 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 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}}

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 (or 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}}.{{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}} 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 spalling. 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 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.}}

Rubber-tyred metros with a central guide rail, such as the Busan Metro, Lille Metro and the Sapporo Municipal Subway as well as rubber-tyred trams have guide wheels.{{Citation needed|date=March 2025}}

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• Adhesive weight
• Cycloid and trochoid
• Wheelset (rail transport)#Conical wheel-tread

{{Reflist}}

• {{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}}