4mm/ft is the most common modelling scale in the UK.
Often associated with Hornby Dublo, Triang, Hornby & Bachmann as well as other well known manufacturers
3.5mm/ft is used in Continental Europe as well as other parts of the world; commonly known as HO scale, information on this is found on the other scales page
4mm/ft comes in a variety of forms:
OO is the most common and uses a track gauge of 16.5mm (16.5 mm does not equal 4ft 8.5in standard gauge.) It was used in early days because it was available for use with continental HO models
Awdry St Nicolas - OO Thomas style layout for the kids to operate