Hybrid systems defy classification in America too, not just in Germany. In San Francisco alone, no one is completely happy that, counterintuitively, Muni’s new Central Subway to Chinatown is made of railway=light_rail even though it required the same tunnel-boring engineering as a bona fide subway system. Despite this and some other subway tunnels, the majority of the Muni system runs aboveground and sometimes in the street. Or that the longer-distance BART lines are tagged as railway=subway. Less than half of the system runs underground, but those parts clearly distinguish it from light rail systems – mostly. At some point, a decision was apparently made that such incompatible railway=* values must never interconnect, and here we are.
These edge cases don’t diminish a need to fill the yawning gap between heavy rail tracks and tram tracks, which might as well be called light rail. This middle category is somewhat defined by grade separation and other characteristics, but there are all manner of exceptions. It’s reminiscent of how we need expressway=yes for a hybrid between a highway=motorway and a surface street, known as “expressways” in engineering jargon, but no one set of criteria can describe every obvious case of an expressway. (And of course, there are street-running heavy rail lines and there are Interstate freeways with the occasional driveway and traffic light.)