Webster's Dictionary defines a model as "a simplified description of a complex entity or process" such as a "computer model" of the circulatory and respiratory systems. The term comes from the Latin root modus, which means "a manner of doing or being; a method, form, fashion, custom, way, or style." More specifically, the word "model" is derived from the Latin modulus, which essentially means a "small" version of the original mode. A "model" of an object, for example, is typically a miniature version or representation of that object. A "working model" (such as that of a machine) is something which can do on a small scale the work which the machine itself does, or expected to do.
The notion of a "model" has also come to mean "a description or analogy used to help visualize something (as an atom) that cannot be directly observed." It can also be used to indicate "a system of postulates, data, and inferences presented as a formal description of an entity or state of affairs."
Thus, a miniature train, a map of the location of key train stations, or a train schedule, are all examples of different possible types of models of a railway system. Their purpose is to emulate some aspect of the actual railway system and provide useful information to better manage interactions with respect to that system. A miniature train set, for instance, may be used to assess the performance of a train under certain physical conditions; a map of key train stations can help to plan the most effective itinerary to reach a particular city; a train schedule may be used to determine the timing required for a particular journey. From this perspective, the fundamental value of any type of model is its usefulness
