What is the purpose of an Annett key?

An Annett key serves a specific function in the rail transport system, particularly concerning the safe operation of track points, also known as switches. The primary purpose of the Annett key is to allow authorized personnel to operate the mechanical locking mechanism that secures the points in either the 'set' position for diverging tracks or the 'normal' position for the main line.

By using the Annett key, rail workers can ensure that the points are correctly aligned for the intended path of a train, thereby preventing derailments and ensuring smooth and safe train operations. This key is essential for safe working practices around points, as it helps maintain control and visibility over the point's state during rail operations.

The other choices do not accurately reflect the function of the Annett key, as it is specifically designed for point operations rather than for signals, level crossings, or switch locks directly. Therefore, referring to the Annett key in the context of operating A frame/points is correct and highlights its critical role in ensuring safety and efficiency in railway systems.