Something or someone that causes a derailment; a device designed to derail trains.
From 'derail' + '-er' suffix. 'Derail' comes from French 'dérail' (de- + rail). The term emerged in the 19th century with the development of railways.
Derailers are safety devices placed on railroad tracks to intentionally derail a runaway train if it's about to cause a worse disaster—it's a weird example of controlled destruction saving lives!
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