In September 1968, along with the AC electrification of all lines on the Tohoku Main Line, the section between Ou Main Line and Fukushima-Yonezawa's Itaya Pass was switched from DC electrification to AC, and the section between Yonezawa and Uzen-Chitose was also AC electrified at the same time. -The Senzan Line between Uzen-Chitose has also become AC electrified, and the ED78 for the sub-trunk line gradient line section has appeared.
The ED78 has specifications that match the line standards of the Senzan Line so that it can be used in common with the Senzan Line and the Ou Line, and it is a large body equipped with an intermediate bogie due to the increase in equipment dedicated to graded line sections even though it is a D type.
Corresponding to the steep slopes of the Itaya Pass on the Ou Line and the Sakunami Tunnel on the Senzan Line, it was the first AC electric locomotive to be controlled by a thyristor along with its brother EF71, and was equipped with a power regenerative brake.