Which signal is symbolized by 'X'?

The signal represented by 'X' is indicative of an interlocking signal. Interlocking signals are crucial in railway systems as they ensure the safe routing of trains through junctions and crossings. They are designed to prevent conflicts between trains by controlling train movements based on the operational status of tracks and signals ahead.

The key function of an interlocking signal is to convey information about whether a route is clear for a train to proceed safely or if it needs to stop. In essence, 'X' symbolizes a critical aspect of train operations, emphasizing safety and efficiency by linking multiple signals and switches within a system to work cohesively.

The other signal types represented in the choices, such as automatic block signals, clear signals, and caution signals, serve different purposes. Automatic block signals are used to maintain safe distances between trains, clear signals indicate that the track ahead is clear, and caution signals alert the engineer to be prepared to stop or slow down. However, none of these accurately capture the function and significance of an interlocking signal represented by 'X'.