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.
1974 LEGO Sets
In 1974, LEGO released several new sets and themes, including the LEGO Universal Building Set, which included a range of new bricks in different shapes and colors. 1974 was also the year that LEGO introduced the LEGO Technic theme, which focused on building complex, mechanically-themed models such as cars, airplanes, and construction equipment. The Technic theme introduced new elements such as gears, levers, and motors, which allowed for more realistic, functional models. Other notable sets and themes released by LEGO in 1974 included the LEGO Space theme, which expanded upon the Space sets released in previous years, and the LEGO Trains theme, which introduced the first LEGO electric trains.