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
1977 introduced the Expert Builder series—precursor to Technic—using beams, axles, and gears for mechanically rich models. Town-style sets broadened everyday subjects with houses, shops, and vehicles. Dedicated Space and Castle themes were still a year away, but elements and styling hinted at their imminent arrival.