Editing High-speed rail (section)

=== International Union of Railways definition ===
The [[International Union of Railways]] (UIC) identifies three categories of high-speed rail:<ref name="Pyrgidis">{{cite book |last=Pyrgidis |first=Christos N. |url={{google books |plainurl=y |id=e5ymCwAAQBAJ}} |title=Railway Transportation Systems: Design, Construction and Operation |date=21 April 2016 |publisher=CRC Press |isbn=978-1-4822-6216-2}}</ref>

* '''Category I:''' New tracks specially constructed for high speeds, allowing a maximum running speed of at least 250&nbsp;km/h (155&nbsp;mph).
* '''Category II:''' Existing tracks specially upgraded for high speeds, allowing a maximum running speed of at least 200&nbsp;km/h (124&nbsp;mph).
* '''Category III:''' Existing tracks specially upgraded for high speeds, allowing a maximum running speed of at least 200&nbsp;km/h, but with some sections having a lower allowable speed (for example due to topographic constraints, or passage through urban areas).

A third definition of high-speed and very high-speed rail<ref>{{cite journal |last1=Nikolaos |first1=Demiridis |last2=Christos |first2=Pyrgidis |date=2012 |title=An Overview of High-Speed Railway Systems in Revenue Service Around the World at the End of 2010 and New Links Envisaged |journal=Rail Engineering International |pages=13–16 |issn=0141-4615}}</ref> requires simultaneous fulfilment of the following two conditions:<ref name="Pyrgidis" />
# Maximum achievable running speed in excess of {{Convert|200|km/h|0|abbr=on}}, or {{Convert|250|km/h|0|abbr=on}} for very high-speed,
# Average running speed across the corridor in excess of {{convert|150|km/h|0|abbr=on}}, or {{Convert|200|km/h|0|abbr=on}} for very high-speed.

The International Union of Railways prefers to use "definitions" (plural) because they consider that there is no single standard definition of high-speed rail, nor even standard usage of the terms ("high speed", or "very high speed"). They make use of the European EC Directive 96/48, stating that high speed is a combination of all the elements which constitute the system: infrastructure, rolling stock and operating conditions.<ref name="GeneralDefinition-EU" /> The International Union of Railways states that high-speed rail is a set of unique features, not merely a train travelling above a particular speed. Many conventionally hauled trains are able to reach {{Convert|200|km/h|0|abbr=on}} in commercial service but are not considered to be high-speed trains. These include the French [[SNCF]] [[Intercités]] and German [[Deutsche Bahn|DB]] [[Intercity (Deutsche Bahn)|IC]].

The criterion of {{Convert|200|km/h|0|abbr=on}} is selected for several reasons; above this speed, the impacts of geometric defects are intensified, track adhesion is decreased, aerodynamic resistance is greatly increased, pressure fluctuations within tunnels cause passenger discomfort, and it becomes difficult for drivers to identify trackside signalling.<ref name="Pyrgidis" /> Standard signaling equipment is often limited to speeds below {{Convert|200|km/h|0|abbr=on}}, with the traditional limits of {{cvt|79|mph|order=flip}} in the US, {{cvt|160|km/h}} in Germany and {{cvt|125|mph}} in Britain. Above those speeds [[positive train control]] or the [[European Train Control System]] becomes necessary or legally mandatory.