Derail is a verb that is described as "to leave the rails"; as a noun however, derail concerns a safety device placed on the track, usually on a siding, to prevent cars from rolling onto the main line. It may have a cast steel frog that diverts one Wheel up and over the rail to stop a car well short of the clearance point. Some derails consist of a single switch point that is normally left open to stop any moving car clear of the main track.
Ref: 120726/2006-10-01
Alternative Spellings (Synonyms): derails