In an excellent post, Matt Johnson explains what happened after Friday’s White Flint-bound Red Line train found itself on the pocket track just beyond Farragut North, how the electromechanical safety systems did the job they were designed to do, and how a derailer works.
… not only did the derailer prevent a collision or damage to misaligned switches, it also prevented the train from fouling either main track. However, while this event saw the safety system avert potential disaster, it is not clear why a potentially dangerous situation was allowed to progress so far.