1. To effectuate, to go off, or come off the train tracks: The two trains were derailed because they ran into each other causeing a disastrous accident.
2. To depart from the preceding course or procedure: Jane's daydreaming was derailed when the doorbell rang.
3. To cause to part from a specific course or path of direction: The staff members derailed the principle's presentation of the agenda by leaving the room.