Japan's Shinkansen lines are all built to Standard Gauge, because trains are more stable, and can go faster, on wider track. Some other lines in Japan use 1,372 mm (4' 6”) or 762 mm (30”) gauge. But the majority, over 83% in terms of distance, of Japan's railways are built to Cape Gauge, 1,067 mm (3' 6”).

Japan utilizes a "dual-gauge" railway system. The majority of the country's conventional rail network (including most local trains and freight) uses 3 ft 6 in (1,067 mm) narrow gauge. This choice was made in the 19th century to save money on mountain tunneling. However, the world-famous Shinkansen (Bullet Train) network uses standard gauge (4 ft 8.5 in or 1,435 mm). The wider gauge was necessary for the high-speed stability required to travel at speeds over 200 mph. This difference means that Shinkansen trains cannot run on local tracks and vice versa, which is why Shinkansen stations often feel like separate entities from the local "JR" stations. In 2026, Japan continues to use "Mini-Shinkansen" on certain routes, where conventional tracks have been "dual-gauged" (adding a third rail) to allow high-speed trains to reach more remote areas like Akita and Yamagata.