Which statement best defines Interlocking?

• A. A single signal device controlling a switch.
• B. A train order issued by dispatcher.
• C. A temporary track layout for maintenance.
• D. An interconnection of signals and signal appliances such that their movements must succeed each other in a predetermined sequence, assuring that signals cannot be displayed simultaneously on conflicting routes.

Answer: (D)

Interlocking is a safety system where signals and their controlling devices are connected so that their movements follow a fixed, safe sequence. This means once a route is set, the interlocking locks the necessary signals and track equipment in place and prevents other conflicting routes from being cleared at the same time. As a result, signals cannot show proceed on routes that would put trains on a collision course.

The other descriptions miss this coordinated linkage: a single signal device controlling a switch describes just one component, not the interlocked whole; a train order is a dispatch instruction, not a signaling interlock; and a temporary track layout for maintenance is not about interlocking logic.