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.
1978 LEGO Sets
In 1978, LEGO released several new sets and themes, including the LEGO City theme, which expanded upon the city sets released in previous years and introduced new models of public transportation such as buses and taxis.