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”). The name comes from its adoption in 1873 by the Cape Colony (later part of the Union of South Africa).

The width of Japanese trains differs significantly between the high-speed "bullet" trains and the standard commuter lines. Shinkansen (bullet trains) are built to a wider "loading gauge" than most trains in the world, with a standard width of approximately 3.38 meters (about 11 feet). This extra width is what allows for the comfortable 3-2 seating configuration in most standard cars. In contrast, Japan's conventional lines (Zairaisen), which make up the bulk of the urban and regional networks like the Yamanote Line in Tokyo, are narrower to accommodate the older "narrow gauge" tracks (1,067 mm). These trains typically have a width of about 2.8 to 2.9 meters. In 2026, newer models on both networks continue to maximize this space with "open-plan" designs and thin-wall technology to provide more interior elbow room. The difference is immediately noticeable to travelers; walking into a Shinkansen feels like entering a wide-body aircraft, while a standard commuter train feels more like a traditional subway or light rail carriage.