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
1974 saw the Universal Building Set broaden parts variety and color, while large ‘Family’ (maxi) figures and the Homemaker line brought interiors and furniture to the forefront. Early mechanical elements began laying groundwork for later Expert Builder/Technic. Trains continued to evolve on 12V in Europe, refining the system ahead of the 1980s expansion.