Rail transport modelling uses a variety of scales to ensure scale models look correct when placed next to each other. Model railway scales are standardized worldwide by many organizations and hobbyist groups. Some of the scales are recognized globally, while others are less widespread and, in many cases, virtually unknown outside their circle of origin. Scales may be expressed as a numeric ratio or as letters defined in rail transport modelling standards The majority of commercial model railway equipment manufacturers base their offerings on Normen Europäischer Modellbahnen (NEM) or National Model Railroad Association (NMRA) standards in most popular scales.

HO or H0 is a rail transport modelling scale using a 1:87 scale. It is the most popular scale of model railway in the world. The rails are spaced 16.5 millimetres (0.650 in) apart for modelling 1,435 mm standard gauge tracks and trains in HO.