What does the 'Z/D' signal designate?

The 'Z/D' signal designates an automatic block signal. This type of signal is used in railway operations to communicate track occupancy and manage the movement of trains. An automatic block signal is typically activated by the presence of a train in the block ahead, serving to stop any following train from entering the block until it is clear. These signals help ensure safe distances between trains, reducing the chances of collisions and improving the overall efficiency of train operations.

In the context of the options provided, interlocking signals are specifically meant for controlling train movements at junctions and switches, requiring a complex set of mechanical or electronic interlocks to prevent conflicting movements. A stop signal indicates a clear instruction for trains to halt, which is not the function of the 'Z/D' signal. A warning signal typically indicates to the train crew that they should prepare to stop or reduce speed due to conditions ahead, rather than providing an automatic system response based on track occupancy.