To cause a train to leave its tracks, or to cause a plan or project to fail or go off course.
From French dérail, combining de- (off) + rail (from English rail); originally a literal railway term from the 1800s that became metaphorically applied to disrupting any kind of plan.
The term 'derail' perfectly shows how new technologies create new vocabulary—trains were so important to 19th-century life that a railroad disaster became the metaphor we still use for any project that goes wrong.
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