Good questation. It's a Scotch gauge,4 ft 6 in (1,372 mm) track gauge (called "coach gauge" in Japan). That was adopted by early 19th century railways mainly in the Lanarkshire area of Scotland. In Japan,tram lines of Tokyo adopted this gauge in 1903. The use of this gauge extended to other suburban lines that projected through services to the city tram. Although Tokyo has abolished its major tram network, The Keio lines (excluding the Inokashira Line) still use this gauge.