HO scale is the most popular model railroad (model railway) scale, and while "standard gauge", 4 feet 81/2 inches, is also the most popular gauge for HO scale, HOn3 is also quite popular in the US. In the United Kindom however, OO scale is more popular and the three-foot gauge version of that is OOn3.
In other parts of Europe and Japan, HOm (meter gauge) and HOn42, 42-inch gauge are also quite popular. In Switzerland HOm with a center rack rail for cog wheel railroads is also often modeled. Where in Japan HOn42 is often modeled because Japan the standard gauge is 42 inches. The high-speed Shinkansen trains run on "wide gauge", 4 feet 81/2 inches. Yes, that is standard gauge in much of the world, but in Japan, it's considered wide gauge!