What is classified as an interlocking appliance?

An interlocking appliance is a system or mechanism that ensures the safe and efficient flow of train movements through junctions, crossings, or interlockings. The correct answer refers to movable apparatuses that control designated routes. These devices are critical for managing how trains move through complex track systems by allowing or restricting access to specific tracks, thereby preventing collisions and ensuring operational safety.

Interlocking appliances typically include switches, signals, and other components that work together to maintain safe train operations. They can change the train's paths by moving tracks and setting signals to direct trains subtly, which provides a structured method of managing train traffic.

The other options refer to different aspects of rail operations but do not specifically pertain to interlocking appliances. For instance, engines are used primarily for pulling trains, fixed signals provide indication but do not have the movable components of interlocking appliances, and inspection vehicles serve a distinct purpose in maintenance rather than routing trains. Therefore, the essence of interlocking appliances lies in their role in managing routes through movable apparatuses, making the selected answer the most accurate definition of what an interlocking appliance is.