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 expanded its product range with new sets that focused on city life, introducing models of houses, stores, and vehicles. This year also saw the introduction of the LEGO Expert Builder series, which laid the foundation for what would later become the LEGO Technic theme. These sets featured specialized elements such as beams, gears, axles, and connectors, allowing for more mechanically complex builds, including cars, airplanes, and construction equipment.