HO scale model railroads are the most popular size. HO scale has a ratio of approximately 1:87, with track gauges of 16.5 millimeters. It originated in the 1930s but became most popular in the 1950s. HO scale allows for a reasonably sized layout in a small space while still featuring detailed scenery models. While other scales like N scale have even smaller sizes, HO scale strikes a good balance of size and detail. HO scale runs primarily uses two-rail DC power that can be adjusted for speed and direction changes.