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 and accessories. 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 Lego fans, in reference to traditional model railway scales. Lego trains use a nominal gauge of , based on 5-stud track centerlines gauge, corresponding with a circa 1:38 scale. Development The design of Lego trains has developed substantially, with several different systems introduced, with varying degrees of cross-compatibility. The Blue Era (1966-80) Lego trains were first introduced in 1966 with Lego set number 080. The train sets used blue rails, and the first train sets were simply push-along. Set number 115 introduced 4.5 volt battery-operated trains (initially the battery box was handheld, but train sets soon contained a railcar that carried the battery box), and train sets numbered 720 (1969) and up operated on 12-volt electrified rails, introduced in 1969. In 1972, 4.5-volt trains gained a monolithic railcar that carried the batteries and contained both a bottom-mounted stop button to be actuated by signals, as well as a side-mounted lever for manual go/stop/back control and tripping by a track-side pivot. All three kinds (push trains, 4.5-volt battery-operated trains and 12-volt electric trains) existed alongside each other and even allowed for upgrades. The motors were the same size, the push trains used a motor-shaped dummy block of bricks, and all used the same wheel style. These wheels had the same press-fit metal axles as used in the two larger sizes of rubber-tire Lego wheels, which also meant that both 4.5-volt and 12-volt motors were not restricted to use in trains. A push train could be updated to a battery-operated train, and a battery-operated train could be updated to an electric train. Since 1969 the motor housings for 4.5-volt and 12-volt are equal and can be equipped with either a 4.5-volt or 12-volt Bühler motor unit. These motor units were sold separately as a replacement part until around 1990. In or before 1976, the 4.5-volt motor gained a hole for driving the then white cross-axles of the size that would later be known as Technic.