Derail: Dictionary Definitions
verb
1. To cause (a train) to leave the rails. This is the most literal and original meaning of the word.
* Example: The train derailed near the station.
2. To disrupt the normal progress of (something); to cause to fail or be unsuccessful. This is a more metaphorical use of the word, referring to the disruption of a planned course or trajectory.
* Example: The scandal derailed the politician's campaign.
3. To cause (someone) to lose control. This usage implies a loss of mental or emotional stability.
* Example: The news derailed her for a while.
noun
1. The act of a train leaving the rails.
* Example: The derailment caused major delays.
2. The disruption of a plan or progress.
* Example: The project experienced a derailment due to unforeseen circumstances.
In summary, the main meaning of derail is to disrupt or cause something to go off track, either literally or figuratively.