@jon A version of the route-km longest trip that might work is the pair of stations, connectable via scheduled rail service, where the shortest connection (without specifying any vias) is the longest. A first guess would be taking your final route and prepending the leg up from Helsinki. But it’s much harder to do an exhaustive search than geodesic distance, so I’m not about to start writing the article on that one!

@jon If the two Madrid stations are considered separate just because the have different UIC codes, then the same logic would separate München Hbf from München Hbf Gleis 5–10, which would just be absurd.