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.

[1] When not in use, the key is normally held in an Annett's lock that is fixed to the lever or concerned apparatus.

[4] In British usage, the key may be held in a locked container in the signal box that can only be opened when the corresponding signal lever is turned to danger for traffic on the running line; the lever is locked in position until the key is returned.

Alternatively, the key may be attached to the train staff for the section of running line containing the entrance to the siding.