Editing Derailment (section)

=== Derailment following collision ===
If a train collides with a massive object, it is clear that derailment of the proper running of vehicle wheels on the track may take place. Although very large obstructions are imagined, it has been known for a cow [[estray|straying]] on to the line to derail a passenger train at speed such as occurred in the [[Polmont rail accident]].

The most common obstructions encountered are '''road vehicles at level crossings''' (grade crossings); malicious persons sometimes place materials on the rails, and in some cases relatively small objects cause a derailment by guiding one wheel over the rail (rather than by gross collision).

Derailment has also been brought about in situations of war or other conflict, such as during hostility by Native Americans, and more especially during periods when military personnel and materiel<!-- this is the correct spelling --> was being moved by rail.<ref name="denevi">Don DeNevi and Bob Hall, ''United States Military Railway Service America's Soldier Railroaders in WWII'', 1992, Boston Mills Press, Erin, Ontario, {{ISBN|1-55046-021-8}}.</ref><ref name="wolmar">Christian Wolmar, ''Engines of War: How Wars Were Won & Lost on the Railways'', Atlantic Books, 2010, {{ISBN|978-1-84887-172-4}}</ref><ref>{{Cite web |url=https://www.pbs.org/wgbh/americanexperience/features/general-article/tcrr-tribes/ |work=American Experience |title=Native Americans and the Transcontinental Railroad |publisher=[[PBS]] |access-date=26 August 2017 |archive-date=10 March 2017 |archive-url=https://web.archive.org/web/20170310195100/http://www.pbs.org/wgbh/americanexperience/features/general-article/tcrr-tribes/ |url-status=dead }}</ref>