Which category of equipment does not consist of multiple units?

Track cars used for inspection are designed as single units rather than collections of multiple units. They are specialized vehicles that serve specific functions, such as maintaining and assessing the condition of the rail infrastructure. Each track car operates independently and does not rely on a multiple-unit configuration, making them distinct from other equipment that combines units for functionality or operation.

In contrast, cab signals located in engines, multiple-unit passenger trains, and interlocking appliances controlling routes all involve multiple components or units working together to fulfill their operational roles. Cab signals often work in conjunction with various locomotive systems, while multiple-unit passenger trains are by definition composed of several railcars that are linked and communicate with each other. Interlocking appliances, which manage train movements at junctions, also consist of interconnected devices to ensure safe operation. Thus, track cars represent standalone equipment in this context, validating their classification as not consisting of multiple units.