Prototype

A prototype is an original type, form, or instance of something serving as a typical example, basis, or standard for other things of the same category. The word derives from the Greek πρωτότυπον (prototypon), "archetype, original", neutral of πρωτότυπος (prototypos), "original, primitive", from πρώτος (protos), "first" + τύπος (typos), "impression".

On a real railway a prototype would normally be the first of a class or an experimental locomotive such as the BR class D16/1 and class D16/2

In modelling, the prototype is the original article being modelled.