Annett's key

In British, Australian, French, and Swiss railway signalling, an Annett's key is a form of trapped-key interlocking that locks levers or other items of signalling apparatus, thereby serving as a portable form of interlocking. The purpose is to prevent access from sidings to the running line without authority. When not in use, the key is normally held in an Annett's lock that is fixed to the lever or concerned apparatus.