As a long-time (14 years) OSM rail mapper, I rather like @gymate’s diagram, though I agree it is fairly complex for beginner-level mappers. It could suffice for advanced mappers who know rail mapping in OSM well, but for beginners, I’d like to see (here, please) what two diagrams that are nearly identical, but one without relations and areas (which could suffice for our “simple railway station” diagram) and the one above, with relations (as more comprehensive, but no longer really “simple”). Also, maybe “mapped mainly as nodes” vs. not. Somewhere, it should be made clear that some elements are optional and some elements do not apply to “not public_transport” flavored stations. The use of color is good, perhaps this can be extended (for optional elements) with additional colors.
I’d make two small tweaks: for both relations, swap the order of type and its first-line companion so that type=site and type-=public_transport come first. (Because a relation’s type is primary in defining what the relation is). Other than that, IMO it’s an excellent diagram, though, it is no longer “simple.” Maybe for the full, comprehensive diagram, we change the word to “generalized.”