HO Scale (known as OO Scale in the UK) is the most popular Gauge used for Model Railways. HO Scale is one of the several Gauges used by Bachmann, the others being N, On30, O and G Scale. "HO" is the abreviated term for "Half-of O", which literally means "Half-of O Scale". HO Scale rails usually spaced about 16.5 mm apart.
After the end of World War One, several attempts were made to try and produce a Model Rail Gauge half that of O Scale. This was because a smaller scale would've been cheaper to manufacture as well as being cheap enough for the ordinary Family to buy and more suitable for making "Tabletop Railways". As early as 1922, German Manufacturers begun producing Clockwork-Driven locomotives to run on this new gauge, 2 years later, the clockwork locomotives were replaced by electric ones.
In the 1930s, many manufacturers started to produce HO Scale Models due to the economic pressures caused by the Great Depression, Marklin was one of the many Companies that begun producing HO Scale Models in this period. However, HO never gained much popularity until the 1950s and the Gauge's popularity soon skyrocketed in the 1960s to the point were HO had overtaken O Scale in popularity.
In 2002, Bachmann released their first Thomas & Friends items, giving birth to the range we now know today.