Any real-life object that's modeled is considered a prototype. Examples of prototypes include locomotives, freight and passenger cars, structures, trackwork, and even an entire railroad.
Many modelers set their layouts in a particular span of time. For example, a modeler may choose a period when steam engines hauled most trains. If the year is 1948, then all locomotives, rolling stock, structures, and vehicles should look like those used during the late 1940s. Period modelers aim for this sort of historical accuracy.
Most American railroads changed from steam engines to diesels between 1945 and 1960; thus, this 15-year period is often called the transition era.
The local freight trains that stopped to pick up and drop off cars at small towns and industry sidings are known as peddler freights but are also sometimes called locals or way freights. A peddler freight was usually short (10 to 15 cars) and was assigned lightweight, versatile locomotives that could handle switching at individual sidings as well as hauling a train on the main line.