There are several words which would do.
According to Wordnet, original, archetype, and pilot can all be used in the sense of something that serves as a model or a basis for making copies. Pilot won't work; it's normally used when a small scale model is created before the full scale vehicle, but you're interested in the opposite circumstance.
Google Translate offers model, example, and paragon as translations of the German word Vorbild you asked about in your comment. Paragon means a perfect example of something, and you are not looking for a word with that limited sense.
Here are some sentences about model trains which use original, archetype, model, and example to mean something that serves as a model or a basis for making copies. Note that model can mean either the the original or the copy.
- Pictured is a 1:8 scale replica of the original locomotive designed by Matthias N. Forney.
- The archetype for today's modern model trains is the Birmingham Dribbler.
- The model for this electric train is the "Train Eclair de luxe": the first Orient Express.
- This HO gauge caboose is an example of the typical crew cabin of the 1970's.