In 1972 LEGO released the 4.5V and 12V train sets as part of the Blue Era which was named because of the rail color. These train sets used additional conductor rails mounted between the standard 4.5V rails and 12V motors contained in the same housings as the 4.5V motors, but with additional metal contacts attached to the bottom of the housings.
1977 LEGO Sets
In 1977, LEGO released several new sets and themes, including the LEGO City theme, which introduced models of city life such as houses, stores, and vehicles. 1977 was also the year that LEGO introduced the LEGO System, which was a standardized system of bricks and connectors that allowed for more flexible and versatile building. The LEGO System included a wide range of bricks in different colors and sizes, as well as new elements such as windows, doors, and roofs. Other notable sets and themes released by LEGO in 1977 included the LEGO Space theme, which introduced new models of spaceships and alien minifigures, and the LEGO Castle theme, which expanded upon the castle sets released in previous years.