O scale is a scale commonly used for toy trains and rail transport modelling. Introduced by German toy manufacturer Märklin around 1900, by the 1930s three-rail alternating current O gauge was the most common model railroad scale in the United States and remained so until the early 1960s. In Europe, its popularity declined before World War II due to the introduction of smaller scales.

A toy train is a toy that represents a train. It is distinguished from a model train by an emphasis on low cost and durability, rather than scale modeling. A toy train can be as simple as a toy that can run on a track, or it might be operated by electricity, clockwork or live steam. It is typically constructed from wood, plastic or metal. Many of today's steam trains might be considered as real ones as well, providing they are not strictly scale or not enough detailed ones in favor of a robustness appropriate for children or an inexpensive production.