Considering that Thomas is a cartoon recreation of a British Railways E2 class locomotive, see if you can find plans for the E2.

Barring that, I doubt anyone will care about the accuracy of the model, so just make something close to it and paint it like Thomas. Even the models that are out now don't always conform to each other (the Trackmaster line tends to be longer than the other models, for example; the Lego Thomas sets had a different cab roofline...)