Seth has always been very interested in railway systems. since he was young he was fascinated with types of trains, train routes, train museums, and reading about the different rail systems that run in different countries. seth has a well- developed for railway systems.

I think the correct term to fill in the blank would be schema. Seth has a well-developed schema for railway systems. A schema is a representation of an idea, object or a theory by means of an outline, diagram or a model. Since Seth already is very knowledge about railway systems, it is given that he would be have already a representative model in his mind.