Lego Trains is a product range and theme of the construction toy Lego, which incorporates buildable train sets. The Lego Trains theme became a sub-theme of Lego City in 2006. Products in the range have included locomotives, tracks, rolling stock, stations, signal boxes, and other track-side buildings. The theme is popular among adult fans, as well as children, and has spawned international associations and conventions. The train system is sometimes referred to as 'L-gauge' among fans, in reference to traditional model railway scales. Lego trains use a nominal gauge of 37.5mm, 5.5mm wider than O gauge, derived from a centreline gauge of 40mm (five lego studs).