As early as 1940, the Japanese Government Railway (JGR) was planning to lay a 4'8.5" Shinkansen (New Main Line) that would permit high-speed rail service on the Tokaido route (Tokyo-Yokohama-Shizuoka-Nagoya-Kyoto-Osaka). Express passenger trains would have been pulled by streamlined 4-8-4 steam locomotives, designated as Class HD53, which were similar to the Asia Express streamlined Pacific (4-6-2) locomotives being used in Manchuria. Based on careful study of British, European, and American locomotives, they would have been capable of exceeding 100 miles per hour.
World War II prevented these plans for a steam-powered Shinkansen from being realized, but much of the initial research and basic concepts were eventually applied to the Tokaido Shinkansen that opened in 1964.
But what if the original idea came to be despite the war, or even after the war?