While the Shinkansen (Bullet Train) is the fastest way to travel between Osaka and Tokyo, it is built for speed rather than sightseeing, often passing through sound barriers and tunnels. For a truly scenic experience, you should consider the "Limited Express Hida" combined with the Shinkansen, or the "Sunrise Izumo/Seto" overnight sleeper train. The Hida route takes you through the Japanese Alps to Takayama, offering stunning views of river valleys and mountains, before connecting back to the coast. Alternatively, the Limited Express "Shinano" from Nagoya to Nagano offers some of the best mountain scenery in Japan. For those who want to stay on the main line but see more, the "Kodama" Shinkansen makes more frequent stops, allowing for better views of Mount Fuji near Shizuoka (sit on the left side when heading to Tokyo). While there is no single "slow scenic train" that covers the whole distance directly, "rail-hopping" via local lines like the Tokaido Main Line allows you to see the coast and countryside at a much more leisurely and picturesque pace.