A device or mechanism used to intentionally derail a train to prevent it from entering a certain area or to stop it in an emergency. The railway workers installed a derailer on the track to ensure the train did not enter the construction zone.
A component of a bicycle gear system that moves the chain from one sprocket to another, allowing the rider to change gears. The cyclist adjusted the derailer to ensure smooth gear shifting during the race.
A person or thing that causes something such as a plan, discussion, or process to deviate from its intended course; a disruptor. Ignoring the meeting agenda, the chief derailer steered the conversation toward unrelated grievances.