Which of the following best describes a 'Train'?

A 'Train' is best described as multiple-unit cars coupled with an engine because this definition encapsulates the primary function and structure of a train in railroad operations. A train consists of several cars, which can include passenger, freight, or other specialized vehicles, connected together and pulled or pushed by a locomotive or engine. This configuration allows the train to transport goods or passengers efficiently over various distances.

In contrast, the other choices do not accurately represent a 'Train.' An inspection vehicle refers specifically to equipment used for track inspection rather than a standard train configuration. A fixed signal for block limits pertains to signaling practices in rail operations, which indicate movement authority and track status but are not a train itself. Similarly, a random number from the Operations Control Center does not relate to the physical structure or definition of a train; it pertains instead to operational data management. Thus, the choice describing multiple-unit cars coupled with an engine clearly aligns with how a train is characterized in the context of railroad systems.