What is an arrangement of signals and appliances that must operate in sequence?

An arrangement of signals and appliances that must operate in sequence is known as interlocking. Interlocking systems are designed to prevent conflicting movements through an arrangement of tracks, ensuring trains are routed safely and efficiently. This system coordinates signals, switches, and other devices to ensure that the rail operations adhere to a predetermined sequence for safe passage. For instance, a signal must show "stop" if a switch is not aligned correctly for a train's intended route. This sequence effectively prevents accidents and ensures that trains operate within set safety parameters.

The other choices refer to related but distinct concepts. Block limits define the sections of track that a train occupies and indicate where train movements can occur, but they do not focus on the sequential operation of signals and appliances. Train control refers to systems that manage train movement over a network, often incorporating speed limits and distance monitoring but is broader than just signal sequentials. Finally, a signal circuit involves the electrical or mechanical components that control signals; however, it does not encompass the overall arrangement and sequence operation that interlocking provides.